[PiP2] Only save reentry state on expand

We should not be saving reentry state on
remove PiP or dismiss PiP. The latter creates
the issue where PiP snap fraction puts it
in the center of the display at the bottom.

Bug: 381017000
Flag: com.android.wm.shell.enable_pip2
Test: Drag to dismiss, enter app, go to PiP
Change-Id: Ie9de88c39f8a199e8aa2e4e7eee6d8d30415dd8c