Add a perf test for picture raster cache (#45050)

This will catch issues like
https://github.com/flutter/flutter/issues/44252, and this test is
inspired by the `list_demo` sample app in
https://github.com/flutter/flutter/issues/43083#issue-509586473

This is tested locally on a Moto G4 before and after the fix
https://github.com/flutter/engine/pull/13710

The `average_frame_rasterizer_time_millis` of this test drops from
~5.4ms to ~4.9ms after that fix.

dev/devicelab/manifest.yaml

diff --git a/dev/devicelab/manifest.yaml b/dev/devicelab/manifest.yaml
index 3aefda7..ab4a895 100644
--- a/dev/devicelab/manifest.yaml
+++ b/dev/devicelab/manifest.yaml
@@ -167,6 +167,12 @@
     stage: devicelab
     required_agent_capabilities: ["mac/android"]
 
+  picture_cache_perf__timeline_summary:
+    description: >
+      Measures the runtime performance of raster caching many pictures on Android.
+    stage: devicelab
+    required_agent_capabilities: ["mac/android"]
+
   cubic_bezier_perf__timeline_summary:
     description: >
       Measures the runtime performance of cubic bezier animations on Android.