Check for PIPable activities after moving task to back.

When we move a task to the back, there is a possibility that the
previous stack won't be put into a paused state if it's in a
multi-resume environment (e.g. freeform windows). If that's the case
and the activity is eligible for PIP, we pause the task so that PIP
mode can be triggered via onUserLeaveHint.
We pause the task instead of using ATM#requestPictureInPictureMode
because the latter is not able to handle the two scenarios we want to
support on minimize:
1. If the app wants to enter PIP -> enter PIP
2. If the app doesn't want to -> put the app to stopped
Using requestPIPMode works for 1. but won't put the app to stopped
when the app decides not to enter PIP. Pausing the task explicitely
does handle both cases correctly, as it triggers onUserLeaveHint and
only puts the app to stopped if the app did not enter PIP mode.

Also, don't call on resumeFocusedStacksTopActivities when in a
multi-resume environment. There is nothing to resume, and the focused
stack is already updated when moving the task back.

Bug: 142680249
Bug: 141156846
Test: In an environment with freeform window, try:
1) Minimizing a PIPable activity - sees it enters PIP
2) With two windows in freeform, try switching focus and see they
perform as expected
3) Minimizing an activity that supports PIP but doesn't want to enter
PIP on onUserLeaveHint - it gets STOPPED

Change-Id: I59811a56a954d32458d741b8f44ccd37f5729094