[lensoverlay] Update text selection to spec

This cl moves to using full line bounding boxes instead of highlighting
each word individually. Additionally, this change allows us to add a
margin to each word bounding box to allow for easier text selection.

Bug: b:330711553, b:336802432
Change-Id: I633d8d137b4850b95f12064779c6d3cbae5903d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5484406
Reviewed-by: Duncan Mercer <mercerd@google.com>
Reviewed-by: Juan Mojica <juanmojica@google.com>
Auto-Submit: Duncan Mercer <mercerd@google.com>
Commit-Queue: Juan Mojica <juanmojica@google.com>
Cr-Commit-Position: refs/heads/main@{#1292571}