Mark non-flaky tests as such (#18927)

dev/devicelab/manifest.yaml

@@ -129,7 +129,6 @@ tasks:
       Checks that `flutter run --release` does not crash.
     stage: devicelab
     required_agent_capabilities: ["mac/android"]
-    flaky: true
 
   platform_interaction_test:
     description: >
@@ -266,7 +265,6 @@ tasks:
       Checks that the module project template works and supports add2app on Android.
     stage: devicelab
     required_agent_capabilities: ["linux/android"]
-    flaky: true
 
   flutter_gallery_instrumentation_test:
     description: >
@@ -282,7 +280,6 @@ tasks:
       Tests the `flutter attach` command.
     stage: devicelab
     required_agent_capabilities: ["linux/android"]
-    flaky: true
 
   # iOS on-device tests
 
@@ -431,7 +428,6 @@ tasks:
       Measures performance of running flutter test.
     stage: devicelab
     required_agent_capabilities: ["linux/android"]
-    flaky: true
 
   technical_debt__cost:
     description: >