Add dialog for testing motion detection.

This is accessible through long press on the developer screen title.
Just a question so that I don't forget to ask tomorrow. It's not clear to me *why* we have this addedDelay.  The raw mPrefMotionChangeTimeWindowMs seems like it should be good enough.  There doesn't seem to be any case where we use the raw value that the user has set without adding this additional 2 second padding.

We'll sort this out in IRC tomorrow - I'll codify the reasoning in a test.