Override modules should always override source modules

Since overrides are implemented as variants of the source module, the
override module should not be replaced with prebuilts of the source
module even when the prebuilt is preferred.

Test: go test ./apex
Change-Id: I26e97f700276e7beaf6d1bd61b164f11d57a5e09

android/override_module.go

@@ -253,6 +253,15 @@ type overrideBaseDependencyTag struct {
 
 var overrideBaseDepTag overrideBaseDependencyTag
 
+// Override module should always override the source module.
+// Overrides are implemented as a variant of the overridden module, and the build actions are created in the
+// module context of the overridden module.
+// If we replace override module with the prebuilt of the overridden module, `GenerateAndroidBuildActions` for
+// the override module will have a very different meaning.
+func (tag overrideBaseDependencyTag) ReplaceSourceWithPrebuilt() bool {
+	return false
+}
+
 // Adds dependency on the base module to the overriding module so that they can be visited in the
 // next phase.
 func overrideModuleDepsMutator(ctx BottomUpMutatorContext) {