Cameraserver: Use multiple hwbinder threads

Some interactions between camera service and hwservicemanager require
multiple threads to execute without deadlocks, such as calling
getService() from within an onRegistered() callback.

Increase thread count to 3 to accomodate.

Also switch the order of listener registration and legacy provider
addition back to what it was originally.

Test: No deadlock when camera service is restarted
Bug: 35096594
Change-Id: I6def961d5765958fef284c0a1820e903abc851ef