Implement Settings UI for Screen zoom preference

Also cleans up unnecessary 0s and moves tint to the proper place in
relevant vector drawable XML files.

Preview layout is still WIP pending UX input, but I wanted to get this
CL in place so that we can test/demo the feature ASAP.

Bug: 22450672
Change-Id: Ic3f1500006c763df99fba2d1d16782b89d6fcae1