Simplify MediaRouter2Manager.Client's lifecycle

This is a cherrypick of commit 3aa151259cc138f7ab1de355fae87004d09ac1db.

Test: atest MediaRouter2ManagerTest
Bug: 239926177
Change-Id: I363dc3918465de7fdb74f833fae78f46bde72a37
Merged-In: I363dc3918465de7fdb74f833fae78f46bde72a37
1 file changed