[ActivityManager] Ensure alive process is not killedByAm.

Symptom: Unable to kill by Am again and will be skipped updating its oom adj.

Root Cause:
A restarted process will reuse original process record.
The flag killedByAm will keep previous state.

Solution:
Reset the flag killedByAm to false when the process is started.

Note: Found another similiar patch If95137d91939cc44882ad2813131bcde0edd0c1b

Change-Id: I59a86648ca8d0aed4c489d92751af120aae5ef90