[ActivityManager] Reset killed flag at attachApplicationLocked

A restarted process will reuse original process record.
If only reset KilledByAm flag, not reset Killed flag.
When app process have been killed by AMS can skip  unnessary ANR,
but not catch lowmemorykiller in AppErrors.appNotResponding()

Change-Id: Ie4653edf4252797eaeefbf9a4a35ac00cfa857d8
Signed-off-by: yuanhuihui <yuanhuihui@xiaomi.com>