audio policy: improve accessibility volume

Fix some regressions caused by the introduction of accessibility
stream type and strategy in M.

- change the way ACCESSIBILITY usage maps to a given strategy: always map
to strategy ACCESSIBILITY but implement a specific device selection for
strategy ACCESSIBILITY in getDeviceForStrategy(). This makes sure that
accessibility prompts always use ACCESSIBILITY stream type.

- optimize getDeviceForStrategy() to avoid reloading the list of outputs
and devices from the observer when called recursively.

-  make sure that all volume APIs also take into account ACCESSIBILITY stream
when called for MUSIC stream. Generalize the concept of matching strategies
for straqegies that should be considered equivalent for volume control.
Modifications in:
- initStreamVolume()
- setStreamVolumeIndex()
- isStreamActive()
- getDevicesForStream()

Bug: 25067903
Bug: 25616373
Change-Id: Id8c7dd3231f79572b278407bc73186fc42dcc028