Notification icon: hammer and sickle, vector

Notification icon: hammer and sickle, vector

Try to placate newer Androids by changing the icon to match the
guidelines from
https://developer.android.com/guide/practices/ui_guidelines/icon_design_status_bar

Since the shield looked odd, anyway, move to a proper hammer and sickle,
as obtained from Wikimedia and incorporated under public domain.  See
https://commons.wikimedia.org/wiki/File:Hammer_and_sickle.svg .

See https://github.com/cmukgb/ctfws-timer-android/issues/6