Import the keyfile to the private no_backup folder

[alanhu-dev]

Is your feature request related to a problem? Please describe.
The keyfile can be safer in storage, prevent the other app (like file manager) to steal it.

Describe the solution you'd like
Import the keyfile to the private no_backup folder.

Additional context
The idea is from Keepass2Android.

[J-Jamet]

The file is stored wherever you want on the application providing the file, depending on the choice of application you make : https://github.com/2bllw8/anemo
Will be done in FileSync : Kunzisoft/FileSync#7

Duplicate : #1157 #133 #1376