Cherry pick Move startIsolatedProcess implementation in ActivityManagerService DO NOT MERGE

This CL addresses the comments in CL 512432, moving the implementation
of startIsolatedProcess in the outer ActivityManagerService class and
making the entry point in the ActivityManagerInternal sublcass just
a proxy method.
This change also addresses a potential NPE, due to startProcessLocked
returning null in some failure cases.

Original BUG:16403706
Original Change-Id: I21eff88c23221653f552cfc171647a839e42a802

Bug: 16723226
Change-Id: I4af9fd7eea10b11e06a5c37d8160c220376b3ed1