Pin code protect restricted content settings

[andreib1]

• I checked, but didn't find any duplicates (open OR closed) of this issue in the repo.
  NB: there is something vaguely similar, but it's rolled into a whole bunch of more complicated changes
• I have read the contribution guidelines given at https://github.com/TeamNewPipe/NewPipe/blob/HEAD/.github/CONTRIBUTING.md.
• This issue contains only one feature request. I will open one issue for every feature I want to request.

User Story:
As a Parent
I want an optional pin code to better protect the age restriction settings
So that it is safe(r) to distribute to a younger audience.

Please :)

Description
Currently during lock-down, a lot of kids schools are distributing educational activities on YT. This is not great, as allowing access to the YT app gives too much freedom to browse inappropriate content, and can provide odd suggestions as to what to watch next. NewPipe is a great alternative; The filter switches in NewPipe allow me to set age restriction, but my kids can turn it back off again unimpeded. A switch on either the whole settings activity or just the UI elements for restricted content would partially mitigate it (at least make it harder to reset).

Wider Benefits
I'm sure this would be popular amongst parents and increase application adoption.

[XiangRongLin]

Pin code for age restricted settings alone won't make any difference, because currently it's broken anyways
#5006 (comment)

[rocketinventor]

The "Safe Mode" toggle does work and there is no reason why it shouldn't be lockable through some method.

[rocketinventor]

@XiangRongLin I just tested with the video URL from #4869 and the settings 100% do work. So this is an issue.