Reset PiP leash crop upon remove

When dismissing PiP by sending it to back
we are also resetting bounds back to parent bounds
in Core, which means the syncTransaction updates the
crop of the PiP task's layer to those of new bounds.

Hence, we should reset crop to initial bounds in startTx
as we are fading the PiP task out, otherwise, we might see
parts of the PiP not rounded out and visible outside of the normal crop.

Bug: 442813722
Flag: EXEMPT bugfix
Test: manually repro the steps in the bug
Change-Id: I5c62727c9526d7b897c354b1c257621debe418af