AudioPolicyManager: fix music volume following alarm

Using stale device selection for accessibility strategy
from cache was causing the wrong device to be returned
by getDevicesForStream() for music stream.
This caused speaker volume to be changed while playing
music over headset after playing an alarm over headset + speaker.

Bug: 29518506.
Change-Id: I06617976e209f6483ea0df1d0a6acb05de7c5cba