Give users the choice of disabling biometrical if it is turned off in the device

[jesmrec]

Look at this scenario:

1. User enables biometrical in ownCloud app. Of course, it was already registered in device.
2. User removes the biometrical from device Settings

At this point, the biometrical is enabled in the app (hidden), and disabled in device. Every time the user opens the app, he will be noticed:

that can not be clear for all kind of users.

As it was suggested here: #54 (comment), the user can be alerted to keep it enabled or disable it in the moment the app detects it is not enabled in device.