Mark flutter_gallery_instrumentation_test as flaky (#18887)

See #18879.

dev/devicelab/manifest.yaml

@@ -275,6 +275,7 @@ tasks:
       test can run on off-the-shelf infrastructures, such as Firebase Test Lab.
     stage: devicelab
     required_agent_capabilities: ["linux/android"]
+    flaky: true
 
   flutter_attach_test:
     description: >