Bluetooth: Use "Disable" in-band ringing instead of "Enable"

* Given that in-band ringing is enabled by default on supported devices,
  the corresponding development settings should be rephrased from
  "Enable in-band ringing" to "Disable in-band ringing" to hint that it
  is enabled by default
* This also gets rid of special logic to check that option in
  initialization
* Add strings to reflect this change as well

Bug: 65383086
Test: make, try toggling the preference and verify whether in-band
ringing works or not

Change-Id: I29f91c7d12c725b12452ec163b75051ff28f500d
Merged-In: I29f91c7d12c725b12452ec163b75051ff28f500d