Add PROPERTY_VOIP_AUDIO_MODE to android.telecom.Call.

This property is used to report the voip audio mode requested by the
underlying connection.  This is useful for ringing calls and held calls
where the current system audio mode will not necessarily match what the
Connection would use.

Test: Added and ran new CTS tests.
Bug: 110540796
Change-Id: I8c26db4b35de4d40d7b76b3c1adc93223afa104b