Add Snapshot tests

[benpious]

There's a couple of options for this:

1. Generate snapshots using android studio, compare against them with low tolerance.
2. Generate with Cyborg
3. "Snapshot" path output instead of images themselves. Has the advantage of being immune to minor changes in the iOS rasterization algorithms, disadvantage is we might miss major changes, and they aren't self documenting. Our representation would be smaller than images though, which is good for repo binary size.