service: Implement IBluetooth::GetState

This CL implements the GetState method of the IBluetooth interface:

   1. A new file, adapter_state.h has been added, which defines the
      STATE_* values from BluetoothAdapter.java in the framework layer.
      This is done this way so that a client library can be built with
      minimal dependency on core service code (e.g. Adapter).

   2. Implemented the GetState method.

   3. Refactored the fake BluetoothInterface such that multiple unit
      test modules can make use of it by centralizing fake behavior
      to hal/.

   4. Added new commands to client/.

Bug: 23227962
Change-Id: Id403094f6fe34cd5cef0577f319309169f74ddf3