Namely, instead of making it lower priority than readerhighlight and
hoping for the best, make it higher priority, as it should (ReaderFooter
itself is *above* Reader, after all), with a sane set of fallthroughs:
* No footer
* No hold-to-skim
* Held outside the footer (but inside the footer tap zone)
This made the crap workaround from #7466 unnecessary, and actually fixes
the behavior in PDFs (because readerhighlight will match the *physical*
page, which may include off-screen content) and ePubs w/r eclaim bar
height (in which case, there's actual content behind the footer that
readerhighlight could have matched on).
Fix#7697
8d3aacbac3