Keep media sessions for stopping user until the stopping is completed

If a user is quickly *switched back*, the user can be started again
without cleaning up its processes, while the onStopUser() is called in
the middle. It's because onStopUser() is for 'stopping is started' but
it may be canceled if the user is started again quickly.

Bug: 154965421
Test: Manual test on AAE emulator with Android-Q, and Pixel 3a XL with Android-R.
Change-Id: I6e51f865dd9e97edd8a465eeba02966b242525b2