hterm: put hidden page up/down buttons outside of <x-screen>
hterm: put hidden page up/down buttons outside of <x-screen>

The <x-screen> element is usually the element that has the focus. Currently, we
put the a11y page up/down buttons in it. This causes ChromeVox to announce
something like "disabled button page up disabled button page down" every time
the user opens a new terminal or switches to a terminal window. This prompt is
not very useful and can be confusing because the buttons are not visible until
they are focused. Also, ChromeVox is sometimes confused and starts to announce
the buttons in the middle of reading the screen content.

This CL put the buttons outside of <x-screen> so that the buttons are only
announced when they are selected.

Bug: 910440
Test: unittests, manual tests with ChromeVox
Change-Id: I03079ff9ab5d845a977f3e36cb2b540c7a37623c
Reviewed-on: https://chromium-review.googlesource.com/c/apps/libapps/+/1975890
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

chromium/hterm,chromium/hterm,chromium/hterm,chromium/hterm