[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] Ensure consistency behavior when a background activity brings another existed activity to front.

Symptom: ANR occurs on previous activity.
Root Cause:
In KK, when a background activity starts another existed background activity (bring to front),
if current focused stack is not the same as the stack of target starting activity,
it will still resume the top of target stack, even the top activity on the target stack may not the same as target activity.
And it will result incorrect focus, press back key will send to previous stack's top then popup ANR on previous activity:
"Reason: Waiting because no window has focus but there is a focused application".

By original code comment, it looks 'bring to front' should not happen in this issue case.
// If the target task is not in the front, then we need
// to bring it to the front...  except...  well, with
// SINGLE_TASK_LAUNCH it's not entirely clear.  We'd like
// to have the same behavior as if a new instance was
// being started, which means not bringing it to the front
// if the caller is not itself in the front.

If the caller and target are in the same stask, it will just deliver new intent without changing task order (the same behavior as JellyBean).
So the patch concept is just to avoid to use target stack to resume top when caller and target are in different stack.

Solution: Do not allow to resume another stack top if non-top activity try to bring existed activity to front.
It may not be a good solution, just a reminder for the issue case.

Reproduce steps:
Assume A, B, C are different app tasks.
When the application stack is like:
  Top C
      B
      A

 #Case 1: Home is foreground
  A starts B with NEW_TASK, C will resume, focus still stays at Home, and window order does not update.
  Then press back key or volumn key will result ANR on Home.

 #Case 2: App is foreground (Resumed activity is C)
  A starts Home, Home will resume, focus still stays at C, and window order does did not update.
  Then press back key or volumn key will result ANR on C.

Change-Id: If05070123b248e2335791e43a4d4ddee6db11d84
Do not show Home behind full screen activity

When ensureActivitiesVisibleLocked goes through foreground activity
stack and reaches non-fullscreen activity, it sets showHomeBehindStack
variable to true.

If there is a fullscreen activity behind, showHomeBehindStack remains
unchanged, which causes Home application to be displayed anyway.
In this case user will see a fullscreen activity and Home activity
simultaneously.

To fix the issue we set showHomeBehindStack to false when we reach
fullscreen activity in the activity stack.

This was made visible by the following commit:
446ef1de8d373c1b017df8d19ebf9a47811fb402

Change-Id: I535c1283a4e26f5cf606375b837d4b7195324af0