audio policy: engine configurable: improve accessibility volume

This is the counter part patch of 28d09f06
for configurable engine verion based on the parameter framework.

It solves:
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.

Bug: 25067903
Bug: 25616373

Change-Id: Ie674e6bfb5d1b25f71a9fd31cd2405c263b1732b
Signed-off-by: François Gaffie <francois.gaffie@intel.com>