Add API to allow apps more control over instantiation

Let them control construction of standard manifest elements (like
activities) through Application. This will allow for things like
dependency injection and class loader magic.

Test: manual
Bug: 62534409
Change-Id: I5fc2a7ec52bb0e232966704d16f94a192d033267
Merged-In: Iaa2215b225c3b97f4e9ac809dfe499f98c7ad5f1