Fragment for MainActivity?

Since this activity is quite sparse we should just have it as a fragment pushed by the scheduling activity (or vice versa, I don't have all of the designs so I'm not sure which would make more sense)--creating activities takes a lot of resources so we should avoid having many of them.

For the sake of merging in all pending changes before the semester's end, feel free to make an issue for this and not change it now.

_Originally posted by @jmf373 in https://github.com/cuappdev/pear-android/pull/3_

Start by reviewing MainActivity and the scheduling activity, then compare them with the available app designs. The issue does not identify which component should become a fragment or define a concrete completion condition, so clarify the intended navigation and design before changing code.

索引モデルが issue の本文から書いたものです。