exoplayer: call validate from init_resources

wa/workloads/exoplayer/__init__.py

@@ -141,6 +141,10 @@ class ExoPlayer(ApkWorkload):
                 return files[0]
 
     def init_resources(self, context):
+        # Needs to happen first, as it sets self.format, which is required by
+        # _find_host_video_file
+        self.validate()
+
         ensure_directory_exists(self.video_directory)
         self.host_video_file = self._find_host_video_file()