Unmark flutter_gallery__back_button_memory as flaky (#33861)

dev/devicelab/manifest.yaml

diff --git a/dev/devicelab/manifest.yaml b/dev/devicelab/manifest.yaml
index 581bbeb..9392670 100644
--- a/dev/devicelab/manifest.yaml
+++ b/dev/devicelab/manifest.yaml
@@ -526,7 +526,6 @@
       Measures memory usage after Android app suspend and resume.
     stage: devicelab
     required_agent_capabilities: ["linux/android"]
-    flaky: true
 
   analyzer_benchmark:
     description: >