Update widget picker row logic to resolve cropping

Using pixels instead of cell spans for calculating the widget picker row
binding logic.

Before: https://screenshot.googleplex.com/3uXX2m7xWh3om5t
Now: https://screenshot.googleplex.com/C593mrQfqTahNrQ

Test: Manual
Fix: 269790954
Change-Id: Ib591a6f6d3cf8b72766e0714b9bc8ceb86b2ba4f