[java] Restore ServiceManager#getService() to return IBinder

This fixes a crash in 3p libraries.

A new API ServiceManager#getService2() has been introduced to
work with the Service enum type.

Bug: 354674329
Bug: 355187769
Test: Run the demo app in b/354674329 and check it works
Change-Id: Ie7f5556b48f87546e1b427d5408120acd9aeea9f