Add aac vbr support for net_test_stack

The original a2dp stack test only supports aac cbr. Check the aac vbr
status in front of the related code section and then test different local test codec info and capability based on aac vbr status.

Tag: #feature
Bug: 170368057
Test: atest net_test_stack
Change-Id: I8dcada94eebe88d02a348d78a7ec8dce463c8600