Code review comment for lp:~mir-team/mir/android-input-transport-scatter-send-event

Ok. Since we apparently intend to do this correctly in the not-too-distant future, and we're capable of breaking protocol at this level at will, we might as well shove a simple
uint8_t mac[20];
into the relevant structures and preserve the blittability at the cost of mac size changes being a protocol break.