Simplify MediaRouter2Manager.Client's lifecycle This is a cherrypick of commit 3aa151259cc138f7ab1de355fae87004d09ac1db. Test: atest MediaRouter2ManagerTest Bug: 239926177 Change-Id: I363dc3918465de7fdb74f833fae78f46bde72a37 Merged-In: I363dc3918465de7fdb74f833fae78f46bde72a37