[ActivityManager] Reduce report ANR on wrong activity.

Symptom: ANR report on wrong activity.

Root Cause:
  KK changed resume behavior that will not update focus when only resume,
if the activity blocked, it may report ANR on previous focus.
  By original concept, it will try to correct the ANR target,
but the stack of waiting(waitingVisible=true) activity may
different with current top stack.
  If it gets key dispatch timeout, mResumedActivity and mPausingActivity
of its stack will be null becuase it is not top stack.
Then it is unable to change ANR target to the real no response activity.

Solution:
 Use focused stack to get the real culprit.

Reproduce steps:
1.Launch an Activity X from launcher, press home key.
2.Launch X from launcher again, X blocks(sleeps 15sec) in onResume, press back key in the beginning of blocking duration.
3.ANR dialog shows launcher is no response.

Change-Id: I99416ad91e349096f995990f2240a97616fbaf28