Fix process state of top app while device is sleeping

The assumption of commit 550ec7d7 is wrong. The top app record
is still needed for assigning PROCESS_STATE_TOP_SLEEPING while
device is sleeping.

Because this change reverts 550ec7d7, to fix original problem
(171060345) that top app is not updated if there is an order
change of activity while device is sleeping, a new invocation
of updateTopApp is added for the case both mResumedActivity
and prevTopActivity are null. So the process of the next top
non-resumed activity can still be set as top app.

Bug: 159104503
Bug: 171060345
Fixes: 171383274
Test: ActivityTaskManagerServiceTests#testUpdateSleep
  CtsStatsdAtomHostTestCases:ProcStateAtomTests#testTopSleeping
  CtsAppTestCases:ActivityManagerTest#testKillingPidsOnImperceptible

Change-Id: I9e6d530b3be9a73f281a8e1dfa135259853a380c