Match socket buffer sizes between audio HAL and BT server

Adjust our mixer buffer size to be one quarter that of
the socket buffer size for quadruple buffering.

Increase socket buffer size from 20*512 to 28*512 to
smooth out variability in data draw from AudioFlinger.

Bug: 28286313
Change-Id: I8a9ca9e1f4639a0724cfe126acc670c2058cb0fb
(cherry picked from commit bf5d6c8d)