Added OSM copyright string

Moved strings from code to the English and German strings.xml files.
Added a comment to the layout file of the map about where to find the
code that sets the strings.

Was quite a hassle to find that string...
Should have used the windows search from the beginning instead of looking through each .xml file ^^
(ended up being hidden in the mapfragment)

Pls the check the code, since I never wrote apps myself for Android it is likely that I made a mistake.
(I'm not sure whether I can set the @string stuff in the mapfragment and whether it is good style and I'm not sure whether I can use a "-" in the string's name (for F-Droid))