[Merge] lp:~phablet-team/mir/track-plug-state into lp:mir

[Kevin DuBois]

Review: Approve

in my reading of hwc spec, we're specifically guaranteed that checking the display config of a device is allowed (<groan>)
https://android.googlesource.com/platform/hardware/libhardware/+/master/include/hardware/hwcomposer.h#492

If it is a problem on this device though, this doesn't look bad, although I might follow up to solve the problem at a higher level and to add tests (this class is just supposed to be a wrapper)
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https://code.launchpad.net/~phablet-team/mir/track-plug-state/+merge/285854
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