Reduce the audio A2DP HAL output audio stream buffer size

Reduce the audio A2DP HAL output audio stream buffer size
(per constant AUDIO_STREAM_OUTPUT_BUFFER_PERIODS) from 4 to 2.
After the change, the output buffer size is double the buffer period.

Bug: 35849921
Test: A2DP streaming with various codec settings. Testplans/83305.
Change-Id: I2a15d468e748a03b481bf2117903ab06cddd02a3
(cherry picked from commit 5e9f56940e1865d16892639e4c9144a65547626e)