Popovers without interactive content shouldn't need partial interest

A change was made recently to the interest invokers explainer, which
makes the "partial interest" concept apply only to target popovers
that contain keyboard focusable elements. If the popover doesn't contain
something keyboard focusable, then "partial interest" is effectively
the same as "full interest". Except that all partial interest targets
have a UA stylesheet rule that shows how to get it to "full" interest,
which is a nuisance when there's no difference.

Implementing this was a bit more tricky than it sounds, because it
isn't possible to check focusability on a `display:none` subtree:
nothing in that tree can possibly be focusable. So the new logic adds
a `kPotentialPartialInterest` state, which is only around between
showing interest and showing the popover. Within `showPopover()`, if
the invoker has `kPotentialPartialInterest`, then once the popover is
showing, the focusability of the descendants is tested, and either
partial or full interest is selected. If the popover show operation
is cancelled for any reason, we go back to `kNoInterest`.

Bug: 326681249
Change-Id: I058f5e9b4955ab664e899e87143d709e932b911e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6509692
Commit-Queue: Mason Freed <masonf@chromium.org>
Reviewed-by: Joey Arhar <jarhar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1457703}