Make it possible to register/unregister players as a stack

Similar to TaskOrganizer, this supports being able to
register/unregister transition players in a lifo manner.

This will enable integration tests to register a transition
player temporarily and then unregister to restore the
original state.

Bug: 341009477
Test: Existing tests
Change-Id: I011095a35e1c4ea14c3a8fa1abf3576041338705