Use validation instead of two-way binding

This is insane, but it appears to be working. We essentially store
things in a separate class for editing, and then commit it back at a
given time.

This business with onViewStateRestored in both TunnelEditorFragment and
in TunnelDetailFragment is buggy and likely wrong.

In general TunnelEditorFragment should probably be rewritten. The
relationship with the changed name is not clear.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

app/src/main/java/com/wireguard/config/Attribute.java

@@ -54,6 +54,8 @@ enum Attribute {
     }
 
     public static String[] stringToList(final String string) {
+        if (string == null)
+            return new String[0];
         return string.trim().split("\\s*,\\s*");
     }