Hearing Aid State Machine: Only use getCurrentState to get connection state.

* In some race conditions, Hearing Aid Service will get different Hearing
Aid connection state form getCurrentState and mConnectionState.
* Only use getCurrentState to get Hearing Aid connection state.

Bug: 120607552
Test: Build Pass,
runtest bluetooth -c com.android.bluetooth.hearingaid.HearingAidServiceTest
runtest bluetooth -c
com.android.bluetooth.hearingaid.HearingAidStateMachineTest

Change-Id: I6e805f9bd230f98b4c1d0a1eb2540f11173618be