"Add field" footer logic

- Instead of an explicit "+" button to add a new field (which
also allows adding multiple empty fields), only show a
"add field" footer to the KindSectionView if there are no
existing empty fields in the section.
- Make the whole footer a touch target and display a
custom message according to the DataKind mime type

Bug: 4363151
Change-Id: Ib7cef5ed5aaefa5b1269f95ed8fe89e7dd742f36
9 files changed