[PiP2] Update size spec before enter PiP

Size spec updates are triggered when display change
happens and we are in PiP, or when PiP aspect ratio changes.

However there might be cases where size spec is left untouched.
For instance, say we enter PiP with the default aspect ratio,
exit PiP, then display change happens (updates the cached layout but not
size spec). If we re-enter PiP at this point with the same aspect ratio,
with the current logic, size spec will remain the same.

Instead we wanna updaate the size spec no matter what upon entry,
before calculating the entry destination bounds.

Bug: 396522512
Flag: com.android.wm.shell.enable_pip2
Test: ABTD run v2/android-crystalball-eng/health/microbench
  /launcher/main/launcher-swipeupapptopip-jank-suite

Change-Id: I48b7736d8609ebe74395bfdb66c7e4bacf71d646