android build: Treat Kotlin warnings as errors, in release builds

In particular this applies to the new `tools/check android`.

A StackOverflow question that had a couple of helpful answers:
  https://stackoverflow.com/questions/34562200/how-do-i-make-the-kotlin-compiler-treat-warnings-as-errors

Author: gnprice

android/app/build.gradle

@@ -74,6 +74,16 @@ android {
     }
 }
 
+tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).configureEach {
+    // Compile Kotlin with `-Werror`... but only in release builds, so that it
+    // doesn't get in the way of quick local experiments for debugging.
+    //
+    // The string-searching makes this a bit of a mess, but it works.
+    // Better would be if we can add this to android.buildTypes.release above;
+    // but on a first attempt that didn't work (it affected debug builds too).
+    kotlinOptions.allWarningsAsErrors = name.contains("Release")
+}
+
 flutter {
     source '../..'
 }