Don't save the default size as reentry state

Fixes issue where the "default" bounds of PIP can be calculated
differently even though the PIP has never been resized. This is
caused by a combination of changing the aspect ratio of PIP and
trying to restore the "reentry" state that is just the default
state.

Instead, we'll only calculate the PIP bounds using the reentry
size if the size was actually changed by the user, otherwise
we'll always calculate using the default size.

Bug: 169379707
Test: atest com.android.wm.shell.pip
Change-Id: I724b167870407f7f37eb9c8a587a98c7972441a4