Prevent NullPointerException on AdapterService.mRemoteDevices

Makes sure that mRemoteDevices is always initialized and not null
even if AdapterService functions are called while Ble is turned
off. Before this if some AdapterService functions were ever used
before Ble was turned on, mRemoteDevices wouldn't have been
initialized and a NPE would occur.

Bug: 21893759
Change-Id: Ia0f41c3d4fbfc6322d49431d1f5021857f018ff0