Add override_android_app module type.

This is a new implementation of overriding module types that makes use
of local variants. With this, product owners can use PRODUCT_PACKAGES to
decide which override module to include in their products.

Bug: 122957760
Bug: 123640028
Test: app_test.go
Change-Id: Ie65e97f615d006b6e9475193b6017ea9d97e8e97

android/mutator.go

@@ -79,6 +79,7 @@ var preArch = []RegisterMutatorFunc{
 	RegisterNamespaceMutator,
 	RegisterPrebuiltsPreArchMutators,
 	RegisterDefaultsPreArchMutators,
+	RegisterOverridePreArchMutators,
 }
 
 func registerArchMutator(ctx RegisterMutatorsContext) {