Hearing Aid: change set/get active device(2/3)

* 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, and each can be null

Test: manual
Bug: 69623109
Change-Id: I1e8541ca5b2030a231bbbea86c71183768a6570f