Fix a bug: unable to start activity when AM is killing the process.

Symptom: No activity was started for startActivity, low repro rate.
Root Cause: Starting activity on a process which was killed by ActivityManagerService.killUnneededProcessLocked() and before receiving AppDeathRecipient of it.
Solution: Choose the flow of starting a new process if ProcessRecord.killedByAm is true.

Change-Id: Ida2639b0fb4631222fc92d65aadc9fd2da637b45