Reset killedByAm flag at attachApplicationLocked

Cherry picked from aosp.

Fixes bug 18593309.

ActivityManager reuses a process record object that killed
by him under some situation. That reused process record inherits
a killedByAm flag unexpectedly.
The killedByAm flag must be reset otherwise ActivityManager can't
judge the process can be killed or not.

Change-Id: If95137d91939cc44882ad2813131bcde0edd0c1b