Do not remove snapshot after task enters PiP

We remove task snapshot if it is visible at the end of transition, and
when Task enters PiP, it leaves no snapshot for that Task. Therefore,
the task thumbnail in Overview would be empty, with disabled actions.

Fixing this by exempt the pip Task from auto snapshot removal.

Flag: EXEMPT bugfix
Video: http://recall/-/aaaaaabFQoRHlzixHdtY/gk4AYIZM3Vx0UIBG5EnVVQ
Bug: 340396323
Test: Swipe to Overview after dismiss PiP
Change-Id: I18d4a80bc82809f1bfa15271090c2e8249eedab3