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

* 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: I48f3388c56434d0c21e09bd8b794e58848cd8794