Avoid race condition in SBC encoder initialization

In the A2DP media task, btif_a2dp_encoder_update() relies on the fact
that btif_a2dp_encoder_init() is called beforehand. Otherwise division
by zero errors will occur.

Since _init() called after the audio stream is opened, and _update() is
called in resonse the the audio stream being opened, a race condition
could ensue where _update() was called before open.

This patch changes the A2DP stream start sequence to ensure the SBC
encoder is initialized before the updated parameters are calculated.

Bug: 17339248
Change-Id: I6ed36a7fff0e4aa672a8866727030681eb4b3dfc