Bluetooth streaming should wait for AVRCP to start.

Currently we are waiting for A2DP to come up before we can start
streaming. This is not ideal since unless AVRCP comes up we cannot show
the UI reliably. That said, for compatibility with devices only having
A2DP we will need to modify this code. For now the A2DP SNK is only
supposed to work with AVRCP TG implementation on the remote.

Bug: b/27744764
Change-Id: I972cf94f5b276d9b0e08315175ae605ccfe6a423