Load static RROs in OverlayManagerService

Only RROs targeting "android" are loaded from AssetManager, and so
others should be loaded from OverlayManagerService.

The RROs targeting "android" would be ingored as they are to avoid
redundant loading.

Bug: 76205654
Test: succeeded building and tested with taimen
Change-Id: Ibeeb3c3f3338a845b2f4c485305636003d9dbd44