[2/2] Lockscreen notifications enhancements

Change-Id: I904aa906c939f52ab8deddb8f0a074337862a3cc
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