AudioService: vol change logs previous index

AudioService events and media.metrics vol events also log
the "old" index on top of the new index

Bug: 239091512
Test: adb shell dumpsys audio | grep -A 50  "volume changes"
Test: adb shell dumpsys media.metrics
Change-Id: I3d2b594f29ccba84d79d061504cf377f168955dd