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
Revert "a2dp: Set the default A2DP bitrate to high quality"

This reverts commit ce6c2405a9272656285019754fd4d4ac223b92bd

The forums are full of users complaining about their A2DP bluetooth connections timing out or stuttering etc. Not all users are affected but there seems to be more than a few.

Change-Id: I0c6e36069c07ace86f2222471f47a88d42d523e3
merge changes from remotes/github/cm-10.1 into master