Format the frequency once.

Summary:
Rather than repeating code every time we output a frequency for a `MemoryEntry`,
format the frequency once, lazily, then use that thereafter.

(Also slightly change the VFO frequency font size, so the whole thing fits, but
ideally this would be dynamic).

Test Plan: Clicked onvarious memories in the emulator.

Reviewers: jcarter, cegeland

Reviewed By: jcarter

Differential Revision: https://phabricator.edgesysdesign.com/D83