Use out pipBounds to save reentry size

Currently, when an out PIP transition happens we combine:
(1) the last user resized bounds, plus
(2) the out pipBounds fraction (mLastReportedBounds)
Using both we create the reentry bounds used to restore
on reentry.

This is actually unnecessary since ag/12307605 and
ag/12309037 both landed. They landed as separate
efforts fixing different issues almost at the same time,
so we didn't realize that getting the last user  resized
bounds was no longer necessary with the safe
mLastReportedBounds (i.e. pipBounds) passed into
onPipTransitionStarted.

The pipBounds argument is equal to the last user resized
size with the current fraction applied, so there's no need
to re-calculate it, we can just use it directly.

Bug: 169373982
Test: enter/exit pip and resize multiple times to verify the
reentry size is correct.

Change-Id: I2517d10bfbc8ca9c15b08b32089bdeeb1dd3aab6