Check parent is attached before restoring from requestFullscreenMode

The mMultiWindowRestoreParent might've been removed while the
requestFullscreenMode task was in fullscreen, so attempting to restore
to it will raise an error in system_server.

This is possible in multi-desks when a task inside the desk root task
requests fullscreen and thus gets moved out of the desk, then if the
desk is removed (e.g. from Overview) there restore parent of the
fullscreen task is no longer valid and we shouldn't attempt to reparent
it there.

Flag: EXEMPT BUGFIX
Bug: 444605677
Test: open two apps in desktop, have one app requestFullscreenMode with
the API, swipe up to overview and dismiss the desk the task was in, go
back to the fullscreen app and make it exit with the API - verify
there's no exception thrown in system server and the system is still
usable.
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:0270e7e424f7836eefe6c6f66e2b06f828884471
Merged-In: I34fd85822b9b5969a10791ec36a92ce7a938e416
Change-Id: I34fd85822b9b5969a10791ec36a92ce7a938e416