[pm] Guard concurrent modifications of mRunningInstalls

PackageManagerService.mRunningInstalls is a SparseArray which is not atomic, and it can be used concurrently by more than one threads. Guard it to prevent data races.

Change-Id: I7b59a4351220f33f94a778e1a622177121fa202d
BUG: 388491757
FIXES: 388491757
Test: builds
FLAG: EXEMPT bug fix