Audio: fix AudioVolumeGroup JNI

Fix volume group conversion from native to java
to make sure the audio source is properly initialized
to the default java value in a playback only context.

Bug: 249966139
Test: atest com.android.audiopolicytest
Test: atet com.google.android.gts.audioservice.AudioServiceHostTest#testAudioVolumeGroups

Merged-In: If6e0b5ba576f3fdf89480205639b1c2a4b722456
Change-Id: If6e0b5ba576f3fdf89480205639b1c2a4b722456
(cherry picked from commit 2c5d09c4)