Currently --browser exact works by providing a --browser-executable and then having Telemetry guess the browser backend from the package name of said executable.

This does not work at all for webview because:

- The "executable" provided here corresponds to the executable of the webview_shell, not of webview itself.

- When the executable is, say, Monochrome.apk, it's still unclear whether the user intends to use the android-chrome or the android-webview-google backend.

This needs to be rethought when revamping the browser-backend and platform mechanism on issue 799950