a2dpsink: Process right amount of data with sbc_encode

sbc_encode() can only process data with at least this->codesize bytes.
When encode_buffer() is called with less then codesize bytes, accumulate
those bytes in a temporary buffer up to codesize length, then SBC encode
them.
Maximum size for SBC buffer is (subbands * blocks * channels * 2) with max
subbands = 8, max blocks = 16, max channels = 2, i.e. 512.
2 jobs for fix_encode_buffer in 54 seconds