A2DP: Enable AAC VBR for A2DP source feature

This CL adds AAC variable bitrate to the source capabilities, and the
software encoder uses VBR mode 5 (vary high bitrate) by default.

Bug: 112325138
Test: atest net_test_stack / IOP test
Tag: #feature
Change-Id: I4271154248af5ea82c3de2c97bb39526e9f904ba