bluetooth: align max renderable block size to frame size

When bluez5 device I/O thread detects it is too slow sending data, allow it to
skip up to max rendered block size bytes which must be aligned to frame size.

Part-of: <pulseaudio/pulseaudio!449>
3 jobs for pr/fix-bluetooth-io-skip-bytes-assertion in 5 minutes and 5 seconds (queued for 2 seconds)