When picking a random sleep image from a set of custom images, compare
the randomly chosen index against a cached value in settings. If the
value matches, use the next image (rolling over if it's the last image).
Cache the chosen image index to settings in either case.
## Summary
Implements a tweak on the custom sleep image feature that ensures that
the user gets a new image every time the device goes to sleep.
This change adds a new setting (perhaps there's a better place to cache
this?) that stores the most recently used file index. During picking the
random image index, we compare this against the random index and choose
the next one (modulo the number of image files) if it matches, ensuring
we get a new image.
## Additional Context
As mentioned, I used settings to cache this value since it is a
persisted store, perhaps that's overkill. Open to suggestions on if
there's a better way.
## Summary
* This PR drastically reshapes the structure of the codebase, moving
from the concept of "Screens" to "Activities", restructing the files and
setting up the concept of subactivities.
* This should help with keep the main file clean and containing all
functional logic in the relevant activity.
* CrossPointState is now also a global singleton which should help with
accessing it from within activities.
## Additional Context
* This is probably going to be a bit disruptive for people with open
PRs, sorry 😞
