frameworks: av: fix bug in the volume management

which can cause volume levels to be applied twice. After applying volume
in the effect chain, the remaining volume needs to be converted back to
floating point before providing to mAudioMixer.

Bug: 15583440
Change-Id: I3c884f95b0fa9ad1b554285b069601c850c54776
Signed-off-by: Glenn Kasten <gkasten@google.com>