Update the AAC bitrate settings

Use AACENC_PEAK_BITRATE to limit the maximum bitrate as limited by
the MTU, and use AACENC_BITRATE to set the encoder bitrate.

Previously, both were set to the same value. That was disabling
the AAC bitreservoir, and eventually affecting the audio quality.

Bug: 35715570
Test: A2DP streaming to various devices
Change-Id: I6adac4889f9b2d7509182df8961ddfada02ee8ac