Bluetooth: Use config value for default max connected audio devices

* The first option in Bluetooth max connected audio devices preference
  should be using system default
* Added template based string array to show system default in the list
  preference and in preference summary when default is chosen
* Reset max connected audio devices property to empty string when
  development setting is disabled or when system default is chosen
* Fix a bug by saving selected value to list preference so that it
  persists when user re-opens development settings

Bug: 64767509
Test: Enable and disable multi-device mode in development settings
Change-Id: I4915f12df0ac0e6f715e44e0df4a3707dde8d1a4
Merged-In: I4915f12df0ac0e6f715e44e0df4a3707dde8d1a4