Add cull opacity perf test to device lab (#25381)

For https://github.com/flutter/flutter/issues/24712

This test verifies that https://github.com/flutter/engine/pull/6923 will speedup the average rasterize time of this test from ~150ms to ~10ms

Please see non-auto-generated files in https://github.com/flutter/flutter/pull/25381/commits/37b21d9fb4756a8d8d12ae0def1d0640740c8818

dev/devicelab/manifest.yaml

diff --git a/dev/devicelab/manifest.yaml b/dev/devicelab/manifest.yaml
index 649df48..f100596 100644
--- a/dev/devicelab/manifest.yaml
+++ b/dev/devicelab/manifest.yaml
@@ -112,6 +112,13 @@
     stage: devicelab
     required_agent_capabilities: ["mac/android"]
 
+  cull_opacity_perf__timeline_summary:
+    description: >
+      Measures the runtime performance of culling opacity widgets on Android.
+    stage: devicelab
+    required_agent_capabilities: ["mac/android"]
+    flaky: true # mark a new test as flaky
+
   flavors_test:
     description: >
       Checks that flavored builds work on Android.