[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