Switch Android page load tests to Browsertime

[davehunt]

The GeckoView page load tests are being migrated to Browsertime so we need to switch all tp6m graphs to show the new results. We don't need to preserve the Raptor results for this target. Let's narrow this to cold page load tests. Note that Browsertime tests do not expose the application in the test name, so we also need to be able to filter based on the application.

[davehunt]

This was partially resolved in #301, however we're currently showing warm page loads. I'll raise a followup to switch these to cold page loads.