A2DP: don't write data to none started AVDT SCB

When using A2DP Software encoding, the stack would send data ready event
to all opened AVDT SCBs in every media tick. Some of those SCBs were
suspended already, and should not consume streaming data from queue.
Here we read data for streaming started SCB only.

Bug: 144668694
Test: Play muisc with multiple A2DP connection
Change-Id: I2498e92c5dc6a07b34abb669b8e029f0e7e2f107