overhauls the sampling display to work for both phones and tablets

"Alright, people, let's do this one last time..."

And by "this" I mean UI improvements for small screen sizes.

The commits I'm merging here apply a bunch of changes, all to the sensor sample activity screen:

• new layout XML files for different screen widths, because there are too many objects on it to reasonably expect us to format them perfectly for both tablet and phone simultaneously
• ConstraintLayouts replace RelativeLayouts, because CL's are easier to modify in the visual editor
• the stream button is next to the interval selector; the sample button is next to the spot text editing box
• small fixes to padding, fonts, spacing, etc. to make it prettier overall

Subjectively, I'm quite happy with the tablet display (both portrait and landscape) and happy enough with the phone display to move on to something else. Related to #11 (closed), an issue we should now review.