Move branding files out of torbutton

From #43182 (comment 3135721).

As part of decommissioning torbutton, we want to move our branding files to somewhere else. Currently all our branding is shared by all versions on both android and desktop, so we just need it in one place rather than multiple branding directories.

EDIT: Actually, it seems we would like these to be different in the future. See !1304 (comment 3137854).