Floss: added an error check in a2dp_aac_encoder_init

In this patch, we added an error check of |pcm_samples_per_frame| returned by
|prepare_context| in |a2dp_aac_encoder_init|.
If |pcm_samples_per_frame| is negative, an error is encountered and
|a2dp_aac_encoder_init| would return immediately.

Bug: 294165759
Tag: #floss
Test: m - None
Change-Id: If28fdaf9d78a1fb38b1f44d7fc6175f1f67627bf