Code review comment for lp:~kdub/mir/display-groups

@andreas, correct, this abstraction change in this MP is expanding to cover a goofy requirement (from a 1 thread/monitor world's perspective at least) that the HWC api has. A multithreaded setup on android would hit a lot of the same timing inconveniences in synchronizing, just down in the HwcDevice code, where it is not in the best position to make decisions to wait or continue.