ActivityManager: don't freeze processes holding file locks

As a temporary measure waiting for the more robust b/176927978 to be
implemented, query the file lock status of processes prior freezing
them, and skip freeze if any locks are held.

Test: verified form the logs that processes holding file locks are not
frozen
Bug: 176928302

Change-Id: Ib7e1715effb63c1b304ce22a9451294fd9f7b10d
Merged-In: Ib7e1715effb63c1b304ce22a9451294fd9f7b10d