Make the new button primary color

[marcoambrosini]

"New" buttons are not primary in other apps because it interferes with the selected navigation elements

However this is not the case for the Files app, since the "new" button is in the app content, we can use the primary color.

[Pytal]

Would this be a simple switch to making the button primary or are there other ideas?

[sorbaugh]

@marcoambrosini as this is relevant for #49171, is there already a concept to work towards here?