Store repeater offset frequency.

Test Plan:
Checked `/data/data/com.edgesysdesign.simpleradio/databases/simpleradio` in a
sqlite3 dump and saw the offset frequency be saved.

Note that this fatals the app until D84 lands, because Frequency can't handle a
negative frequency yet.

Closes T85.

Reviewers: jcarter

Reviewed By: jcarter

Maniphest Tasks: T85

Differential Revision: https://phabricator.edgesysdesign.com/D90