screen_ui: Compute the top and bottom gaps.

We're not actually following the gaps as in the comments. For example,
Nexus 6P is supposed to use 220dp and 194dp gaps (top and bottom
respectively), but the actual numbers are 185dp and 194dp. Because the
animation icon and text sizes don't match the ones claimed (animation:
expected 200dp or 700px, actual 800px; text: claimed 14sp, actual 76px).
The top gap changes (shrinks) as we compute the baselines bottom-up.

This CL switches to using computed gaps: the major UI elements always
stay vertically centered, with identical top and bottom gaps.

Bug: 63093285
Test: 'Run graphics test' on angler/volantis/fugu/ryu.
Change-Id: I3cadbb34f728cf034afa47ac02a6deba8cb6b4e7