bluetooth: refactor code, support multiple adapters

- Provide new dbus interfaces to support multiple
  adapters.

- Adjust config file structure to support multiple
  adapters.

- Refactor code, use signals instead of the bloated
  dbus properties, the new signals include AdapterAdded,
  AdapterRemoved, AdapterPropertiesChanged and
  DevicePropertiesChanged.

- Add dbus interface RequestDiscovery

- Add an virtual state to check the device if is
  connecting.

tower: https://tower.im/projects/8f19f0bf0e754f0b82ef2c24bc230973/todos/a630b2a4cf864baa9c21a4713b7bf803/
tower: https://tower.im/projects/15081ba156b14fa4bd6d9e82f91edff3/todos/2a264f0e1b4c4872a8f8452455734fb3/
Change-Id: Id43b67596f9bb6e53e07b0b2120172e0aa9fa8ba