HFP: synchronize sendDeviceStateChanged()

onSignalStrengthsChanged and ACTION_SIM_STATE_CHANGED trigger
sendDeviceStateChanged() from different threads. It may cause
race condition and send old device state to the headset.
Use "synchronize" to make sure device state changed event
sent in the right order.

Bug: 146400643
Test: manual, on/off airplane mode, connect headset
Change-Id: Ia02b6c2a96b19ab53322e237de458811e070f9a2