Log what is happening during path selection

Downstream, in the proprietary parts of Xamarin.Android, there is some
strange errors occurring, such as:

    Task "InstallPackageAssemblies"
    InstallPackageAssemblies Task
        ...
        AndroidSdkPath: /Users/builder/android-toolchain/sdk
        AndroidNdkPath: /Users/builder/android-toolchain/ndk
        JavaSdkPath: /usr
    Xamarin.Android.Common.Debugging.targets(479,2): error ADB0000:  Error finding Android/Java SDKs [/Users/builder/jenkins/workspace/monodroid-pr/monodroid/tests/runtime/Xamarin.Android.RuntimeTests.csproj]
    Xamarin.Android.Common.Debugging.targets(479,2): error ADB0000: System.InvalidOperationException: Could not determine Android SDK location. Please provide `androidSdkPath`. [/Users/builder/jenkins/workspace/monodroid-pr/monodroid/tests/runtime/Xamarin.Android.RuntimeTests.csproj]
    Xamarin.Android.Common.Debugging.targets(479,2): error ADB0000:   at Xamarin.Android.Tools.AndroidSdkInfo..ctor (System.Action`2[T1,T2] logger, System.String androidSdkPath, System.String androidNdkPath, System.String javaSdkPath) [0x0004b] in <5fd439f4454d4cc18bc8c35d2b4c840c>:0  [/Users/builder/jenkins/workspace/monodroid-pr/monodroid/tests/runtime/Xamarin.Android.RuntimeTests.csproj]
    Xamarin.Android.Common.Debugging.targets(479,2): error ADB0000:   at Xamarin.AndroidTools.AndroidSdk.Refresh (System.String androidSdkPath, System.String androidNdkPath, System.String javaSdkPath) [0x00000] in <9cb77bd2eef54f10a4aa7622ee40470d>:0  [/Users/builder/jenkins/workspace/monodroid-pr/monodroid/tests/runtime/Xamarin.Android.RuntimeTests.csproj]

We are having a lot of trouble even figuring out what is going on,
since the EXACT SAME paths worked earlier in the same build. Earlier
in the log `ResolveSdks` worked just fine!

I'm hoping this logging information will help us troubleshoot this
issue and other scenarios in teh future.

Author: jonathanpeppers

src/Xamarin.Android.Tools.AndroidSdk/Sdks/AndroidSdkBase.cs

@@ -61,13 +61,37 @@ public AndroidSdkBase (Action<TraceLevel, string> logger)
 
 		public virtual void Initialize (string androidSdkPath = null, string androidNdkPath = null, string javaSdkPath = null)
 		{
-			androidSdkPath  = androidSdkPath ?? PreferedAndroidSdkPath;
-			androidNdkPath  = androidNdkPath ?? PreferedAndroidNdkPath;
-			javaSdkPath     = javaSdkPath ?? PreferedJavaSdkPath;
+			if (string.IsNullOrEmpty (androidSdkPath)) {
+				androidSdkPath = PreferedAndroidSdkPath;
+				Logger (TraceLevel.Info, $"Trying {nameof (PreferedAndroidSdkPath)}: {androidSdkPath}");
+			}
+			if (string.IsNullOrEmpty (androidNdkPath)) {
+				androidNdkPath = PreferedAndroidNdkPath;
+				Logger (TraceLevel.Info, $"Trying {nameof (PreferedAndroidNdkPath)}: {androidNdkPath}");
+			}
+			if (string.IsNullOrEmpty (javaSdkPath)) {
+				javaSdkPath = PreferedJavaSdkPath;
+				Logger (TraceLevel.Info, $"Trying {nameof (PreferedJavaSdkPath)}: {javaSdkPath}");
+			}
 
-			AndroidSdkPath  = ValidateAndroidSdkLocation (androidSdkPath) ? androidSdkPath : AllAndroidSdks.FirstOrDefault ();
-			AndroidNdkPath  = ValidateAndroidNdkLocation (androidNdkPath) ? androidNdkPath : AllAndroidNdks.FirstOrDefault ();
-			JavaSdkPath     = ValidateJavaSdkLocation (javaSdkPath) ? javaSdkPath : GetJavaSdkPath ();
+			if (ValidateAndroidSdkLocation (androidSdkPath)) {
+				AndroidSdkPath = androidSdkPath;
+			} else {
+				AndroidSdkPath = AllAndroidSdks.FirstOrDefault ();
+				Logger (TraceLevel.Info, $"Falling back to {nameof (AllAndroidSdks)}: {AndroidSdkPath}");
+			}
+			if (ValidateAndroidNdkLocation (androidNdkPath)) {
+				AndroidNdkPath = androidNdkPath;
+			} else {
+				AndroidNdkPath = AllAndroidNdks.FirstOrDefault ();
+				Logger (TraceLevel.Info, $"Falling back to {nameof (AllAndroidNdks)}: {AndroidNdkPath}");
+			}
+			if (ValidateJavaSdkLocation (javaSdkPath)) {
+				JavaSdkPath = javaSdkPath;
+			} else {
+				JavaSdkPath = GetJavaSdkPath ();
+				Logger (TraceLevel.Info, $"Falling back to {nameof (GetJavaSdkPath)}: {JavaSdkPath}");
+			}
 
 			if (!string.IsNullOrEmpty (JavaSdkPath)) {
 				JavaBinPath = Path.Combine (JavaSdkPath, "bin");
@@ -122,23 +146,29 @@ public string NdkHostPlatform {
 		/// </summary>
 		public bool ValidateAndroidSdkLocation (string loc)
 		{
-			return !string.IsNullOrEmpty (loc) && ProcessUtils.FindExecutablesInDirectory (Path.Combine (loc, "platform-tools"), Adb).Any ();
+			bool result = !string.IsNullOrEmpty (loc) && ProcessUtils.FindExecutablesInDirectory (Path.Combine (loc, "platform-tools"), Adb).Any ();
+			Logger (TraceLevel.Info, $"{nameof (ValidateAndroidSdkLocation)}: {loc}, result={result}");
+			return result;
 		}
 
 		/// <summary>
 		/// Checks that a value is the location of a Java SDK.
 		/// </summary>
 		public virtual bool ValidateJavaSdkLocation (string loc)
 		{
-			return !string.IsNullOrEmpty (loc) && ProcessUtils.FindExecutablesInDirectory (Path.Combine (loc, "bin"), JarSigner).Any ();
+			bool result = !string.IsNullOrEmpty (loc) && ProcessUtils.FindExecutablesInDirectory (Path.Combine (loc, "bin"), JarSigner).Any ();
+			Logger (TraceLevel.Info, $"{nameof (ValidateJavaSdkLocation)}: {loc}, result={result}");
+			return result;
 		}
 
 		/// <summary>
 		/// Checks that a value is the location of an Android SDK.
 		/// </summary>
 		public bool ValidateAndroidNdkLocation (string loc)
 		{
-			return !string.IsNullOrEmpty (loc) && ProcessUtils.FindExecutablesInDirectory (loc, NdkStack).Any ();
+			bool result = !string.IsNullOrEmpty (loc) && ProcessUtils.FindExecutablesInDirectory (loc, NdkStack).Any ();
+			Logger (TraceLevel.Info, $"{nameof (ValidateAndroidNdkLocation)}: {loc}, result={result}");
+			return result;
 		}
 
 		protected static string NullIfEmpty (string s)