[ActivityManager] Fix race condition in pss collection

Symptom:
There has a race condition that two threads are accessing
the mPendingPssProcesses simultaneously. One of the thread
is collecting the process pss by looping the mPendingPssProcesses.
The other thread is requesting to collect pss of all processes,
which clears mPendingPssProcesses and adding processes back.

Solution:
Avoid race condition by adding synchornized protection.