Add option to force-stop app before starting

The previous commit introduced:

    scrcpy --start-app=name

By adding a '+' prefix, the app is stopped beforehand:

    scrcpy --start-app=+name

This may be useful to start a fresh app on a new virtual display:

    scrcpy --new-display --start-app=+org.mozilla.firefox

PR #5370 <https://github.com/Genymobile/scrcpy/pull/5370>