Add onAvailableClockChanged to ClockRegistry

ClockRegistry will call onAvailableClockChanged when the list
of available clocks is changed due to a plugin either connecting
or disconnecting.

Bug: 269169005
Test: atest ClockRegistryTest
Test: Manually validated callback was executing at expected time
Change-Id: If44e833def096148e1fd81bf797d163e6a09601c