If I understand correctly, it can be confusing for a user to enable a setting from a screen which only contains login information.
Nevertheless if this is mandatory and within actual context, I would put it within the white card, between the password field and login button so there is a hierarchy of actions (1. enter pwd, 2. choose setting 3. login and ...
I would say it depends. I think is more about avoiding accidental exits of processes and details. If you consider that this could affect your app consider not showing the bottom bar on second level navigation. Also if you want to place a bottom action bar, for example, it could potentially increase the risk of accidental navigation. So you can show two bars, ...
There are no such specific norms to use any component of Android.
But Yes, There are some usage and anatomy of every component or criteria from the design and Usage perspective.
Bottom navigation should be used for:
- Top-level destinations that need to be accessible from anywhere in the app
- Three to five destinations
- Mobile or tablet only.