Unlock screen: Ask for confirmation after tapping "Delete advanced unlock key"

**Is your feature request related to a problem? Please describe.**
I use the database in read-only mode by default. I tapped the wrong button when trying to switch to read-write mode countless times.

**Describe the solution you'd like**
It's in the title.

**Describe alternatives you've considered**
Move that button into "More options".

**Additional context**
Also, I find the process of adding an advanced unlock key (e.g. fingerprint) rather unclear. Every time I tap the delete button by mistake, I need to figure out how to add the key back.

Start at the Android unlock screen and locate the "Delete advanced unlock key" action and the existing advanced unlock key flow. Add confirmation before deletion, then verify that cancelling preserves the key and that the key can still be removed and added again.

Written by the indexing model from the issue text.