Remove unnecessary imports.
Replace hard-coded strings with values in strings.xml.
Remove unused Done string.
SubjectExtrasFragment: Use borderless buttons.

As suggested by android-lint.
This is what the Android UI guidelines recommend, though I am not
convinced that it looks quite right, because I don't see any
separating lines.
http://developer.android.com/design/building-blocks/buttons.html
http://developer.android.com/guide/topics/ui/controls/button.html#Borderless