Add a mechanism to configure the default A2DP codec priorities

Previously, the relative codec priorities were hard-codec internally.
The new mechanism uses the following configurable resources in
packages/apps/Bluetooth/res/values/config.xml to re-assign the
default codec priorities per device, or to explicitly disable a codec.

 - a2dp_source_codec_priority_sbc
 - a2dp_source_codec_priority_aac
 - a2dp_source_codec_priority_aptx
 - a2dp_source_codec_priority_aptx_hd
 - a2dp_source_codec_priority_ldac

Those values are assigned on startup.
Also, they can be changed per device by using an overlay:
device/<VENDOR>/<DEVICE>/overlay/packages/apps/Bluetooth/res/values/config.xml

Test: Manually streaming to a headset
Change-Id: Ic4da3a51ac73f00cbae731156cb7878c8fadee06