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
a2dp hw: adding a2dp tuning through bdroid_buildcfg.h

This adds possibility to configure a2dp tuning on an
arbitrary sysfs path and arbitrary ON/OFF values.
It's activated when a2dp is being used, and
deactivates when a2dp is stopped.
This way a device can either use an a2dp_tuning sysfs file
or cpu scaling min freq sysfs path with a target
min_freq value for elevation.

Here are two example of the two strategies of usage for
this:
bdroid_buildcfg.h - sysfs tuning file
 #define A2DP_HW_SYSFS_TUNER "/sys/devices/platform/tegra_uart.2/a2dp_tuning"
 #define A2DP_HW_SYSFS_TUNER_OFF "0"
 #define A2DP_HW_SYSFS_TUNER_ON "1"

bdroid_buildcfg.h - scaling min freq file
 #define A2DP_HW_SYSFS_TUNER "/sys/devices/system/cpu/cpu0/cpufreq/scaling_min_freq"
 #define A2DP_HW_SYSFS_TUNER_OFF "0"
 #define A2DP_HW_SYSFS_TUNER_ON "204000"

Change-Id: I2cd94e626eb2a6d04a6e52ddac94fa5773eb09d8