Use UserSwitchObserver to detect switches in the running user

The current implementation relies on listening for
Intent.ACTION_USER_SWITCHED, which can take more than 30
seconds to propagate, making UserSwitchObserver a more
reliable option.

Bug: 242188673
Test: atest mediaroutertest CtsMediaBetterTogetherTestCases
Test: Manually tested casting on secondary users.
Test: Pending CL to verify route availability on secondary users as part of the linked bug.
Change-Id: I67765a671bce590495b73ef7df6bf72ba3c9f03a