Reset killedByAm flag at attachApplicationLocked

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

services/core/java/com/android/server/am/ActivityManagerService.java

@@ -6098,6 +6098,7 @@ public final class ActivityManagerService extends ActivityManagerNative
         app.hasShownUi = false;
         app.debugging = false;
         app.cached = false;
+        app.killedByAm = false;
 
         mHandler.removeMessages(PROC_START_TIMEOUT_MSG, app);