[PWA/NavCapture] Simplifying test pages to only emit one DOM message.

This change simplifies our launch detector more, to only emit one
NavigationComplete message when the page is loaded. This prevents that
message from accidentally be detected by future steps. Note - it then
listens for more launchParams being consumed, which is intentional to
test the 'focus-existing' behavior.

This also fixes the app to have it's start url at the destination page, and not the start page. That can just be launched explicitly on
startup.

Change-Id: I7666d7565cf5b686af76056681d2d1893e4cf878
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5906722
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Auto-Submit: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Finnur Thorarinsson <finnur@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1363697}