VolumeControl: Fix registering more callbacks

With this fix new registered client should get callbacks called with
current values of offset (if exists) and current volume

Bug: 314738219
Bug: 307408418
Test: atest VolumeControlServiceTest
Flags: EXEMPT fix for functionality not yet used
Change-Id: I6a39b787e0d9243206b5f4006906796d9280e7ef