Add a devicelab task that generates sample app screenshots, etc (#10303)

dev/devicelab/manifest.yaml

diff --git a/dev/devicelab/manifest.yaml b/dev/devicelab/manifest.yaml
index d98fa2a..914f4fa 100644
--- a/dev/devicelab/manifest.yaml
+++ b/dev/devicelab/manifest.yaml
@@ -171,6 +171,13 @@
     stage: devicelab
     required_agent_capabilities: ["has-android-device"]
 
+  android_sample_catalog_generator:
+    description: >
+      Builds sample catalog markdown pages and Android screenshots
+    stage: devicelab
+    required_agent_capabilities: ["has-android-device"]
+    flaky: true
+
   # iOS on-device tests
 
   channels_integration_test_ios:
@@ -240,6 +247,13 @@
     stage: devicelab_ios
     required_agent_capabilities: ["has-ios-device"]
 
+  ios_sample_catalog_generator:
+    description: >
+      Builds sample catalog markdown pages and iOS screenshots
+    stage: devicelab_ios
+    required_agent_capabilities: ["has-ios-device"]
+    flaky: true
+
   # Tests running on Windows host
 
   channels_integration_test_win: