Make !BorderContainer splitters wider on iOS and android, so that they can be easily grabbed.   Didn't adjust the !BorderContainer padding or gutter size to match; maybe I should, to be consistent, but the other issue with mobile is limited screen space.   Fixes #16831.

Also adding .dj_ios and .dj_android CSS flags to uacss.js.
add test for !TitlePane as single child of a !ContentPane, refs #16674