Fix checkbox in magnification mode dialog does not have ripple effect

* Move onClickListener to LinearLayout since checkbox has null background in xml
* Rewrite setTextAreasClickListener logic to fit the need: at least one
checkbox need to be checked, and to let talkback announce disabled when
view is really in disabled state.

Bug: 164343865
Test: manual test
Change-Id: Ia5a831febc41f01858908cbdc365cf9a6a1282e3