Introduce notion of active devices.

The list of active devices is populated by the Engine.
The active devices are selected with the following rules:
1) If all of the preferred devices for media are available then they
   will all be used for the output.
2) If not, then the last connected removable device will be used.
3) If there are no removable devices then the default audio policy rule
   will be used.
In this CL, we only add a function to query active devices. In a future
CL we will use this list when opening an output.

Test: dumpsys
Bug: 160352965
Change-Id: I12f9ed24e1404264ad7055f3a7c73b444b33ee1a