Remove hardcoded default user agent from touchscreen.py and static_browser.py

[jandro-air]

Small change to remove the hardcoded user_agents. @constantegonzalez found that it could be reported to internal security as an old browser, in some situations.

Made this a PR b/c not sure how important it is in the first place to change. But secondly, looks like lg_screenshot looks for a default user agent, in which case it might be better to simply update the user agent to something a bit more recent, even if it eventually gets "old" again.

No seeming issues running on the system when manually removing the user agent and running the touchscreen browser.