Initialize security_slider in GV at 4

We never explicitly initialize the security level and we set the default value as 0 in #40125 (closed). Unfortunately, I misread the geckoview code and the "default value" does not mean "we use this value only if the pref does not exist", it means "we use this value until we're told we should use a different value" - this is despite the pref being set in 000-tor-browser.js. In any case, here we are. We should explicitly set the the default as Standard (4).