Add a missing call to btif_av_stream_stop() for SetActivePeer(null)

Without this call, the BtifAv state machine could be left in Started
state, while the A2DP media task timer is canceled.
This internal state inconsistency prevents the proper handling
of Audio HAL commands like SUSPEND - no ACK is returned and
the Audio HAL is blocked.

Bug: b/78901123
Test: Manual - play music to Headset, select "play media to" > This device
      resume music, select "play media to" > Bluetooth Headet

Change-Id: I11b280bea76de02f522e61ebb0a9d7eb5b50f993