[View] Clear Hardware-Layers before system destroys HardwareRenderer​.

Symptom: Sometimes HardwareLayer display is corrupted.

Root Cause: In some cases, ViewRootImpl destroys HardwareRenderer​ without
clearing HardwareLayers in the view hirarchy. If the system trims memory fully
and kill the glContext at the same time, HardwareLayer may corrupt.

Solution: Clear Hardware-Layers before system destroys HardwareRenderer​.

Reproduce steps: Modify the cache's trim memory mechanism to make it easier to
trim fully, HardwareLayers implemented in app will easily be corrupted.
Modification of the cache is not required but make it easier to reproduce.

Change-Id: If57c802cdacf8dc37cbbcc199e74f9482834f981
[ActivityManager] Avoid keeping restarting home when only home activity exists.

Root Cause:
When there is only home activity existed,
updating home apk will call forceStopPackageLocked to finish the activity.
Then activity history becomes empty, then home will be launched,
but its package is still target to close that results a loop.

Solution:
If home activity has been force-stopped, do not stop the same home activity again.

Change-Id: Icff12028d407873c2e6f50a06bcad231b908ccbd
[ActivityManager] Move top task to top in window manager

Symptom:
In certain situations, application starts activity while
it is in background. When an existing background activity
starts a new activity which results to be kept in the same
background task, the new activity won't be resumed.
In that case, the background task (sourceTask) should not be
moved to top in window manager.

Solution:
Move top task of the target stack to top in window manager

Change-Id: Id7a37ea67ce1f80e0c2b5399865c51fd7113deb8
[ActivityManager] Do not finish root activity when reset task

Symptom:
The root activity not always located at index 0 of the task.
For example, the index 1 activity will become the new root
when the original root activity (at index 0) finished.
The new root activity might be finished unexpectedly before
the original root activity actually destroyed.

Solution:
Check frontOfTask to avoid finishing the root activity unexpectedly

Change-Id: I623ab97e9c95c83b3cfe7c9dfc151a291a391ea4
[ActivityManager]: Update home process when home activity resumed

Symptom:
  When switch between two home activities, the home process might not be updated to the correct one. In that case, the home activity might be killed easily.

Root Cause:
  The home process is updated only when a home activity is newly created or being restarted. ActivityManager did not update the home process when simply resume a home activity.

Solution:
  Update home process when home activity resumed

Reproduce Steps:
  1. Install a launcher application, such as Apex Launcher
  2. Press home key to change to Apex Launcher by "Just once" option
  3. Press home key to switch back to original home activity by "Just once" option.
     (The home process is still the Apex Launcher's process because ActivityManager does not update the home process when resuming the original home activity.)

Change-Id: I046279ca7ba851a283ee67ea19202890f7b3f343
[ActivityManager] Avoid mistaking visibility by finishing task.

Sympton:
Next activity only adds to history but does not launch/resume then results ANR.

Root Cause:
In a rare timing, some windows are switched at the same time,
it will cause some finishing records on the top temporarily,
then set startIt to false that skip to resume the real top activty.

Solution:
If all activities in a task are finishing, do not use it to check.
The behavior/checking is the same concept as in JellyBean:
            // If starting in an existing task, find where that is...
            boolean startIt = true;
            for (int i = NH-1; i >= 0; i--) {
                ActivityRecord p = mHistory.get(i);
                if (p.finishing) { // <--
                    continue;
                }

Change-Id: I9d81a7b5182400c52e173da23eee61c74692beee