Mark flutter_gallery__transition_perf as non-flaky (#39580)

One of our linux/android Moto G4 has alerts (emergency, amber, etc.)
turned on. That's probably the cause of the flakiness. I've turned it
off.

Let's mark this as non-flaky and see if it's fixed. If not, I'll move it
to mac/android and see if that fixes the flakiness.

I think the most important thing to do right now is to mark it as
non-flaky so those who really break the test could get a notification.

dev/devicelab/manifest.yaml

@@ -627,7 +627,6 @@ tasks:
       Measures the performance of screen transitions in Flutter Gallery on
       Android.
     stage: devicelab
-    flaky: true
     required_agent_capabilities: ["linux/android"]
 
   flutter_gallery__transition_perf_with_semantics: