Move RootPickerDialog and prohibit adding a new root if there is already a root inside

[mdrlzy]

Move RootPickerDialogFragment here from Navigator
The functionality of the RootPickerDialogFragment and the ArkFilePickerViewModel.pinFile() are the same. In fact, we can add root/favorite from two different places. We need to reuse the general logic and prohibit adding root if there is already a root inside.
We also need to prohibit adding device root as ark root.

[kirillt]

We also need to prohibit adding device root as ark root.

I'm thinking about this again and it seems there could be scenarios when it's ok, e.g. SD card if user doesn't have a lot of data on it. Do you think we should support it?