Mark flutter_gallery_instrumentation_test as flaky (#18887)

See #18879.

dev/devicelab/manifest.yaml

diff --git a/dev/devicelab/manifest.yaml b/dev/devicelab/manifest.yaml
index d780647..87e09ca 100644
--- a/dev/devicelab/manifest.yaml
+++ b/dev/devicelab/manifest.yaml
@@ -275,6 +275,7 @@
       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: >