Marked flutter_gallery_instrumentation_test flaky (#15891)

diff --git a/dev/devicelab/manifest.yaml b/dev/devicelab/manifest.yaml
index 4fb4ebb..6e2de5f 100644
--- a/dev/devicelab/manifest.yaml
+++ b/dev/devicelab/manifest.yaml
@@ -249,6 +249,7 @@
       test can run on off-the-shelf infrastructures, such as Firebase Test Lab.
     stage: devicelab
     required_agent_capabilities: ["linux/android"]
+    flaky: true
 
   # iOS on-device tests