[demoapp] make the Thread Network Settings page scrollable

The page is made scrollable, and moved epskc related text/buttons to the
bottom of the page.

A screenshot of the screen scrolled down:
https://screenshot.googleplex.com/Bx2dFHu3SqpbEri

Bug: 372820898
Change-Id: Ie28345463275a177a3fbf272339912b9ea56e38f
1 file changed