Disable PiP entry when switching tasks via recents

With shell transitions, the recents activity gets brought to
the front. This sets enter-pip-on-task-switch so once we actually
switch tasks, the previous one goes to pip.

In legacy, because of launch-behind, enter-pip-on-task-switch
doesn't get set.

In order to match the current behavior (don't PiP during recents
switch), we prevent enter-pip-on-task-switch into transient
activities. Then, only if the transient-launch is actually
committed, we enable the flag before attempting to pip.

Bug: 220196913
Test: open an auto-pip app. quickswitch to another app.
Change-Id: Ib7d2fa36c74eca9b166a957a907b2681f48e9d0e