mrtest.apk_installed: Add return type hint for get_installed_all

mrtest/apk_installed.py

@@ -5,11 +5,12 @@
 import logging
 import os
 import subprocess
+from typing import Optional
 
 cache_installed_all = None
 
 
-def get_installed_all():
+def get_installed_all() -> dict[str, Optional[str]]:
     global cache_installed_all
 
     if cache_installed_all: