a2dp: Set the default A2DP bitrate to high quality

Allows bluedroid to calculate the proper A2DP bitpool size.
This resolves the main A2DP audio quality issues with Android 4.2,
but does not address the difference in audio output gain/volume as
compared with Android 4.1.

Setting the output volume level to approximately 80% in conjunction
with this change seems to result in comparable audio quality

Change-Id: Ic05c3278ebe9288d00926fab8b7c1a646fd861cd
libbt-hci: include bdroid_buildcfg.h from device repo

This lets device repos override some constants with their bdroid_buildcfg.h if
necessary.

This fixes Bluetooth issues (namely tethering) on endeavoru where
BTHC_USERIAL_READ_MEM_SIZE needs to be set to a higher value.

Change-Id: I8955963cdb2b34f1ba63f8160861699f7284814f