[lensoverlay] Add ability for close affordances to hide overlay.

Close affordances in the overlay should only hide the overlay instead of
closing / ending the entire Lens session when the back to page feature
is enabled. This CL also adds a close button after a selection is made
when the feature is enabled. It is anchored to the top right of the
selection overlay.

Bug: 426641577, 426640433, 426644326
Change-Id: Ib26e1e1b0315cf5a45afd3f73ea7c5bda6895276
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6659173
Reviewed-by: Duncan Mercer <mercerd@google.com>
Commit-Queue: Juan Mojica <juanmojica@google.com>
Cr-Commit-Position: refs/heads/main@{#1478631}