[PiP2]Take stashing into account on display change

Whether we are rotating the display while in PiP or
folding/unfolding a device, we should conserve the stash
state as well.

To do this properly first calculate the snap fraction taking
stash state into account. Then, use that snap fraction with
stash offsets and insets to calculate the proper final bounds.

Bug: 381171053
Flag: com.android.wm.shell.enable_pip2
Test: manually stash PiP then fold/unfold or rotate
Change-Id: Ic885e6c4990f7139d9aca3bb6322e133ff60fedd