Mitigation for mass GC deletion

Delete sessions after 2 seconds from when CC::destroy() is called, to
ensure they get the chance to be re-used without being destroyed but
still get destroyed if they aren't re-used.

This also adds several new unit tests around the functionality to ensure
its thread-safety.

Test: hwuitest
Bug: 300360668
Change-Id: Icc35293ff49e14dc3599fc12d153f70acd29042a