Hearing Aid: change get/set active device (1/3)

* setActiveDevice() returns false in error case, e.g. when the device is
not connected
* add getActiveDevices() instead of isActiveDevice(), which returns a list
that must have two elements: left and right, or empty list on error

Test: manual
Bug: 69623109
Change-Id: I6f872dd88422e9d7e5187603d3ad33628726e1ca