Add MediaSession.finalize to ensure the session is unregistered

Apps may forget to call release(), which risks leaking the
session state in the system which keeps the reference to it for
as long as the app is alive.

This can be fixed by adding a finalize() implementation that
triggers the release if not already done.

Bug: 261882243
Flag: EXEMPT bugfix
Test: new test in same topic
Change-Id: I41afa5bef3b1b90501851ee7875217ef8f268930