Changing BluetoothHeadset APIs from @hide to @SystemApi

For isInbandringing API - it is used in the call journey and would
change the behavior(undeterministic) of the call if not used.

Stop/StartScoUsingVirtualVoiceCall is used in the BluetoothShim layer
specifically for testing and mocking in the Android Apps. Seems
reasonable to keep this as a SystemAPI given that it has no params and
unlikely to be changed.

Bug: 195160939
Tag: #feature
Test: Manual
Change-Id: I02b2e7e5e0be3b462f7c25b655e669e2c7fe47eb