Update app icon and support the round icon of Android 7.1

Besides the current material icon request in the app (#272 #273), I'd like to see an updated app icon as well. In my opinion a white "w" on a black background should be enough to recognize wallabag. And with Android 7.1 in sight, it would be useful to create an round icon as well :)

Start by reviewing the existing material icon requests in issues #272 and #273 and the app's current icon assets. Determine where the Android 7.1 round icon is defined, then update the app icon to the requested white “w” on black design and add the round variant.

Written by the indexing model from the issue text.