[lensoverlay] Use a region query for the zero state contextual search

Previously, initiating a zero state contextual search would send a
predefined text query. This change modifies the zero state flow to
instead perform a region-based query that encompasses the entire
viewport. The side panel is now opened immediately to
show a loading state while the page is contextualized and the
full-viewport region search is performed.

Bug: 449767015
Change-Id: I94b8db25acf506da5bf090a7f8c66cc3c4db8cf2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7021343
Reviewed-by: Duncan Mercer <mercerd@google.com>
Commit-Queue: Juan Mojica <juanmojica@google.com>
Cr-Commit-Position: refs/heads/main@{#1527842}