AVRCP: Change to pass PTS TCs for Vol up/down passthrough.

This change is to pass PTS TCs TC_CT_PTT_BV_02_I and
TC_CT_PTH_BV_01_C which requires DUT to send Vol up and
down passthrough commands. Since we do not have BT test app
enabled for Avrcp Controller, we fake these pass through when
audio adjusts absolute vol using +/- vol key button presses.

CRs-Fixed: 1034579
Change-Id: I4d19baa24d7960278a37633e494185ebd69b5452