Improve notification spam filter code.

- Follow coding style guidelines
- Replace dialog shown on click by adding the relevant information and
  the delete button to the item itself
- Minor layout tweaks
- Make the empty view more descriptive
- Add license headers

Change-Id: Ic685364087d6a093fdbb6a143a9c6ed78596b62b

Conflicts:
	res/values/cm_strings.xml