Move audio.a2dp.default to vendor

Default HAL should go to /vendor/.

Bug: 35907904
Bug: 35979581
Test: Compiled and checked install path. Verified on Sailfish with
$ lsof | grep default. MP3 playback over my BT headset.

Change-Id: I93a341cc96b8d991dea52070f633566bf71f0dc0