audio policy: fix volume control regression

commit 28d09f06 introduced regressions in volume control resulting
in changing some device/stream volumes unexpectedly.

The problem was in the logic consisting in considering streams equivalent for
volume control based on their associated strategy: The rule is wrong because it
causes a volume change on a given stream to cause the same change on all streams
sharing the same strategy. For instance stream SYSTEM and MUSIC share the same
strategy and changing SYSTEM stream volume would change MUSIC volume.

The new rule just considers the stream type when deciding if two stream share the
same volume control.

Also fixed possible calls to getStrategy() with stream PATCH causing asserts.

Bug: 27586382

Author: Eric Laurent <elaurent@google.com>
Date:   Tue Mar 8 10:43:05 2016 -0800

    audio policy: improve accessibility volume

Change-Id: I3fe2a25767319800b0db5fd1ed7c1bd751b04732