Only allow access to log file via explicitly created URIs

Since ContentProviders are public and permissions don't seem to work any
other application could access the log file.  With this token system
only URIs we explicitly created can be accessed.

src/frontends/android/AndroidManifest.xml

@@ -62,6 +62,9 @@
         <provider
             android:name=".data.LogContentProvider"
             android:authorities="org.strongswan.android.content.log" >
+            <!-- android:grantUriPermissions="true" combined with a custom permission does
+                 not work (probably too many indirections with ACTION_SEND) so we secure
+                 this provider with a custom ticketing system -->
         </provider>
     </application>