Store object references along with child session Ids (1/n)

1. Null-check is no longer needed in removeChildSessionId()
   since we never store null in child sessions.

2. Simplify the lock model for we no longer need
   mSessionProvider.getSession() to retrieve a child session which
   must be called outside mLock.

Bug: 162386287
Test: atest StagedInstallTest

Change-Id: I05580968a6ed97647c841f7c47fc1cb1a94ead5a