[ActivityManager] Improve task order of getRunningTasks.

Symptom:
During switching task in same stack, the first result
of getRunningTasks will be the behind stack's top task.
e.g.
 App Task X is starting task Y, the first entry may be home.

Root Cause:
TaskRecord's lastActiveTime is updated when pausing
or resuming. When X task launch a new task Y, Y is
on the top of task history, before X complete pause,
Y's lastActiveTime will be 0 because it is a new task.
Then when comparing the front task with other stack,
other stack will be regarded as the newer one.

Solution:
If the stack is focused stack, give the top task with the last time.

Change-Id: I0adc07608e03d333e0120a0dbc52a0fbbbb12f34