Incompatible APIs were used in the custom layouts and the extra keys
options.
Add @TargetApi annotations to help catch similar issues in the future
with the help of 'gradle lint'.
This separates the layout definitions from the special layouts
(bottom_row, greekmath) and other unrelated files (method, settings).
This is also a more intuitive location for layouts and make the resource
directory easier to navigate.
Under the hood, layouts are copied back into
build/generated-resources/xml.
