Add tasks for gain/lose interest on interesttarget [2/N]

This CL changes the behavior of gaining and losing interest such that
both are scheduled as tasks, optionally with CSS-driven delays. That
not only adds the native ability to control those delays (which, per
the conversation at [1], seems important) but also this resolves
some interesting behavior even when the delays are zero:
 - position the popover on top of the interest invoker
 - show interest, via hover, on the interest invoker
 --> that will invoke the target popover
 - the target obscures the invoker, which immediately triggers the
   loss of interest on the invoker, and that closes the popover

Bug: 326681249
Change-Id: I88d1cdaf975d56b819a0ff93cc8a48c2c5d71649
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6175776
Reviewed-by: David Baron <dbaron@chromium.org>
Commit-Queue: Mason Freed <masonf@chromium.org>
Auto-Submit: Mason Freed <masonf@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1409860}