Add the @Keep annotation to the methods that are called from C++, so they aren't removed.

This way, we don't need to keep proguard rules in synced with the engine dependencies.

The rules we currently tell folks to set are:

-keep class io.flutter.app.** { *; }
-keep class io.flutter.plugin.**  { *; }
-keep class io.flutter.util.**  { *; }
-keep class io.flutter.view.**  { *; }
-keep class io.flutter.**  { *; }
-keep class io.flutter.plugins.**  { *; }

@matthew-carroll could you help with this?