diagnostics: Split app initialization

Tests:

- When a print message is added in low disk space checking method, it is printed
every 3 minutes.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/diagnostics/__init__.py

@@ -64,6 +64,9 @@ class DiagnosticsApp(app_module.App):
                                        **manifest.backup)
         self.add(backup_restore)
 
+    @staticmethod
+    def post_init():
+        """Perform post initialization operations."""
         # Check periodically for low RAM space
         interval = 180 if cfg.develop else 3600
         glib.schedule(interval, _warn_about_low_ram_space)