hwc2: Move callbacks_lock_ and client_connected_ into HWCCallbacks

*Ensure hotplug attempts to the client succeed while avoiding deadlock
caused by Refresh during Hotplug between SF callback handler and HWC
callbacks
*Move client_connected_ inside HWCCallbacks and expose getter
*Update HWC code to use callbacks_ instead of wrappers

Change-Id: I580cec61981ffba2b225ea84894d3b898be25d2d
CRs-Fixed: 2462838
8 files changed