Don't allow MemoryFiles of negative length.

Prevents us from converting a (signed) jint into an
(unsigned) size_t and having horrible things happen.

Change-Id: I0f04e2eb9852ae7fc49b435fd0974f56e86751a4
[ActivityManager] Fix a bug: unable to start activity after starting activities during screen off.

Symptom: Unable to start any activity.
Root Cause: ActivityStack.mPausingActivity() points to a destroyed activity of a died process, so that ActivityStackSupervisor.allPausedActivitiesComplete() always returns false.
Solution: Set mPausingActivity to null in ActivityStack.cleanUpActivityLocked().
Reproduce steps:
    a. Turn screen off.
    b. A background service starts an activity X (in process X).
    c. A background service starts a no-history activity Y (in process Y), but the main thread of Y was blocked.
    d. A background service starts Y 3~4 times --> this causes am_failed_to_pause on X.
    e. Main thread of Y is freed finally --> this causes Y crash for android.view.WindowManager$BadTokenException.
    f. Turn screen on, X is shown on screen, but neither back key nor home key can work because mPausingActivity is Y.

Change-Id: I320b3db407e2d4cc745c8ca22a6e548742234242
[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