compat: enable media devices without legacy gUM

enumeration of media devices is protected by RFP and use of media devices (webrtc which TB doesn't have) requires user actions

• make sure media.navigator.enabled = true (default true in FF)
• make sure media.devices.enumerate.legacy.enabled = false (I think we set this in base browser: default true in FF128)

why

• might help with bad feature detection on websites
  • another example where we should be smarter, instead of just disabling APIs
• removes a FPing diff between MB/TB (JS wise)
  • while we will always have the little-t tor difference, the rest is absolutely achievable and we should aim for that as a healthy thing

99% sure there is zero risk here but 1% of me says ask for @pierov 's opinion 😄