headset: check property before showing ui

Take system property `bluetooth.hfp_volume_control.enabled` into
consideration when performing volume changes to determine
if volume ui should be displayed or not.

Similar check already present in HeadsetClientStateMachine.java

Bug: 348420422
Bug: 404152232
Test: atest BluetoothJavaUnitTests:HeadsetStateMachineTest
Test: adjust volume in call using handsfree while
        `bluetooth.hfp_volume_control.enabled` is false.
Flag: com.android.bluetooth.flags.hfp_volume_control_property
Change-Id: I3cca9f580fb188ab37355d71b4c8a51699082efc