Prevent orphaning of windows when token is removed

When the owner of a window token requests window manager to remove the
token, we were removing it from it's parent which orphaned the token
and all it's windows and prevented them from being accessed from calls
like forAllWindows(). This problem wasn't visible before as the token
windows would still be in the window list, so they can still be accessed
for the exit animation transition which eventually removes them.
We no longer remove tokens from their parent we when are asked to remove
them from the system. We just set WindowToken.setExiting() so the token
can be removed once all its windows are removed at the end of the exit
animation.

Also,
- In AppWindowToken.removeIfPossible() and Task.removeIfPossible(), call
removeImmediately() instead of parent.removeChild() to make sure the
token and its windows are properly clean-up and avoid orphaning if they
aren't.
- Added DisplayContent.reParentWindowToken() for changing the display a
window token is on which is different from DisplayContent.removeWindowToken
which prepares the token and it's windows to be removed from the system.
- Renamed WindowToken.explicit to WindowToken.mPersistOnEmpty which is
what it does.

Fixes: 33098800
Test: bit FrameworksServicesTests:com.android.server.wm.WindowTokenTests
Test: Make sure toast windows don't persist on screen.
Change-Id: I40e0e8832141514b614e79d8e95cd27f24e52366