BluetoothManagerService: Fix: headsetService is unbound abnormally

When bt is turning on, adding proxy is delayed because of
MESSAGE_ADD_PROXY_DELAYED. If a bluetoothHeadset
object is destructed at this time, unbindBluetoothProfileService
will be called, then binding headsetService will be stopped
abnormally since getRegisteredCallbackCount() returns 0.

Change-Id: Ia7f6628ecdc5e4f4b69392658b4f188de6816e7d
CRs-Fixed: 2502607