Tweak DelegatedAdminReceiver API surfaces

* Add nullability annotaitons to new and existing (DeviceAdminReceiver) APIs
* Make onReceive() final
* Javadoc for uid paramater
* Throw exception if method is not implemented

Bug: 123926441
Test: make -j32 update-api
Change-Id: Ib068b1d7f8f5abe903720b8e247c5e00bf8161c2