Check idle activity from all activity container

The home task could be:
TaskHome
 - TaskFragment
   - ActivityB
 - ActivityA

Then forAllLeafTaskFragments won't iterate ActivityA under TaskHome
because it is a leaf Task but not a leaf TaskFragment. So separate
the procedure to forAllLeafTasks + forAllLeafTaskFragments.

Fix: 360946107
Flag: EXEMPT bugfix
Test: atest RootWindowContainerTests#testAllResumedActivitiesIdle
Change-Id: I2bb4bc7d141163d47f16c0e895afa451f46157d5