Add .editorconfig (#6461)

This causes the 'max line length line' in Android Studio to be at 120
chars, which is the same limit as checkstyle uses.

.editorconfig

@@ -0,0 +1,7 @@
+# Settings in .editorconfig should match checkstyle config
+
+root = true
+
+[*]
+charset = utf-8
+max_line_length = 120