Make android_app.stem overridable

override_android_app can set "stem" property to set the APK name.

Bug: 193504286
Test: m (soong test)
Change-Id: I9ad66ea28bd1b030cb2c588d1f070d72e62ab1c7

java/java.go

@@ -1867,6 +1867,7 @@ func DefaultsFactory() android.Module {
 	module.AddProperties(
 		&CommonProperties{},
 		&DeviceProperties{},
+		&OverridableDeviceProperties{},
 		&DexProperties{},
 		&DexpreoptProperties{},
 		&android.ProtoProperties{},