Adapter: Use config value for default max connected audio devices

* Use config_bluetooth_max_connected_audio_devices config value for
  default max connected audio devices
* Override above value using system property if the property is set by
  development settings
* Check and normalize resultant value to permitted range
  [MAX_CONNECTED_AUDIO_DEVICES_LOWER_BOND,
  MAX_CONNECTED_AUDIO_DEVICES_UPPER_BOUND]
* Dump this value to dumpsys

Bug: 64767509
Test: Enable and disable multi-device mode in development settings
Change-Id: Id038d742e9aadfc964b52e406467f4e86a73b73f