Fix issue #17305377: Don't kill process if it still has tasks.

We previously killed a process when one of its task was
swiped away in the recents UI. This had negative performance
implications for apps with multiple tasks in recents. Now we
will only kill the process if there are no more tasks associated
with it.

Changed also removes the need for the
ActivityManager.REMOVE_TASK_KILL_PROCESS since ActivityManager
will now only kill a task process if it process has no out
standing tasks.

Bug: 17305377
Change-Id: Ibc39bb328d13c7eab05c04798c2f14887923d9d4