Adjust mako ui anchor rect and arrow.

Set the mako ui bubble anchor rect and arrow so that it appears
correctly relative to the caret, i.e. try to position so that the top
left of the mako ui is aligned with the caret, but adjust to try to keep
onscreen. (Still appears offscreen if the screen is not big enough, but
we can deal with that later.)

As a drive by, also clean up some of the anchor rect and arrow logic. In
a followup, we will add an option to position the mako ui at the centre
of the screen, which is what we want for the consent screen.

Bug: b:296154580
Change-Id: Ie82c732d4b97e0ca04ec0a1ebdd4c00116618b4e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4856182
Commit-Queue: Michelle Chen <michellegc@google.com>
Reviewed-by: John Palmer <jopalmer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1195120}