Updated Android minimum API level to 21 (Android 5.0)

This is necessary for the window safe area support, and is a reasonable default for SDL 3.0

docs/README-android.md

@@ -16,7 +16,7 @@ https://developer.android.com/sdk/index.html
 Android NDK r15c or later
 https://developer.android.com/tools/sdk/ndk/index.html
 
-Minimum API level supported by SDL: 19 (Android 4.4)
+Minimum API level supported by SDL: 21 (Android 5.0)
 
 
 How the port works