Fix redundant set codec preference to native

- Add logic to check current codec status and only
  set preference codec to native if we expect codec
  change.
- Add unit test for A2dpCodecConfig.java

Bug: 116545163
Test: Change codec in Developer Settings, runtest blueotooth

Change-Id: I6f406e8cd221b6610d059e8bbd80c0bf03092448