Refactor hook handlers from MobileFrontend.body.php to MobileFrontend.hooks.php

This is intended to improve general code organization for improved
readability, codeability, etc.

Also turns all hooks into static methods. There are still two legacy
hook handlers in MobileFrontend.body.php that should be migrated to
MobileFrontend.hooks.php after the following are merged:
Change: I33ed5fea
Change: Ie01bc7f7
Change: I5c1a1c1f

Change-Id: I97099fb6232b542edbd853c460c864969602cff0