[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
Merge "[ActivityManager] Do not finish root activity when reset task" into kitkat