Don't try to auto-pip if there's already a pip

Was mistakenly not checking. Legacy just forbid this, so
do the same with shell.

Also, move when we reset track-count in wmcore. This is
not really consequential, but without it the logs are
a little less clear.

Bug: 280227595
Test: Swipe-to-home an auto-pip app while there is already
      a pip.
Change-Id: I95204309614bac523abdf9787ad668026d5d0401