Correct the value for BT_RC_NUM_APP for Multi-A2DP / Multi-AVRCP

The original value of BT_RC_NUM_APP was incorrectly increased from
1 to 12 for the purpose of supporting up to 6 AVRCP connected devices.

The new BT_RC_NUM_APP value is corrected to 6:
  BT_RC_NUM_APP -> 1 -> 12 -> 6 (AVRCP-related - MaxDevices)

Test: Manual - connected to multiple headsets and selected each
      headset to play audio.
Bug: 70350399
Change-Id: I53e350c2bf1717b1f031a7a265adf3417cf3c513