android [nfc]: Merge drawable-v21/ into drawable/

Our minSdkVersion is no less than 21 -- in fact it's 28 -- so the
app only ever runs on devices that will pick the v21 version over
the baseline version.  So simplify that variation away.

This may break local Android builds in an existing worktree;
it seems like the Android Gradle Plugin is inappropriately caching
where it expects to find this file and gets confused when that copy
goes missing.  To fix, clear the relevant cached data by running:

  $ tools/gradle --rerun-tasks \
      :app:bundleDebugResources :app:bundleReleaseResources

Author: gnprice

android/app/src/main/res/drawable-v21/launch_background.xml

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <!-- Modify this file to customize your launch splash screen -->
 <layer-list xmlns:android="http://schemas.android.com/apk/res/android">
-    <item android:drawable="@android:color/white" />
+    <item android:drawable="?android:colorBackground" />
 
     <!-- You can insert your own image assets here -->
     <!-- <item>