[dashboard] switch from Stocks to Gallery

[yjbanov]

Measuring the Stocks app is not as useful as measuring the Gallery app. Additionally, the Stocks app relies on data from the Internet to function, which makes it flaky.

[sethladd]

cc @HansMuller

[github-actions]

This thread has been automatically locked since there has not been any recent activity after it was closed. If you are still experiencing a similar issue, please open a new bug, including the output of flutter doctor -v and a minimal reproduction of the issue.