Turn off flaky indicator for flutter_gallery__back_button_memory and...

Turn off flaky indicator for flutter_gallery__back_button_memory and flutter_gallery__memory_nav (#58838)

After updating the GBoard on the devicelab machines, these tests are no longer flaky.

dev/devicelab/manifest.yaml

@@ -752,14 +752,12 @@ tasks:
       Measures memory usage after repeated navigation in Gallery.
     stage: devicelab
     required_agent_capabilities: ["linux/android"]
-    flaky: true
 
   flutter_gallery__back_button_memory:
     description: >
       Measures memory usage after Android app suspend and resume.
     stage: devicelab
     required_agent_capabilities: ["linux/android"]
-    flaky: true
 
   flutter_gallery__image_cache_memory:
     description: >