AudioService: fix volume code path with a null audio device

The VolumeGroup synchronization with the VolumeStream calls
into setStreamVolume without a device.

Bug: 302751899
Test: atest android.media.audio.cts.AudioManagerTest
Test: atest com.android.audiopolicytest.AudioVolumeGroupTest
Change-Id: I27051bc8bfda8df3ff4b6998c19f40a936121680