Allow the use of the app despite the "app obscured" dialog if not all apps can be disabled to obscure Cryptomator

### Summary

By default, Cryptomator is disabled if the app is obscured which has valid reasons
> The reason for Cryptomator not working is that the user interface of Cryptomator is obscured. Whenever another app obscures Cryptomator, it could intercept the input done to Cryptomator or display a false UI tricking the user into doing stuff he does not want to do. For security reasons, Cryptomator is disabled by default when obscured. The Android documentation contains [some more details](https://developer.android.com/reference/android/view/View.html#Security).

Read more about it in our docs: https://docs.cryptomator.org/en/latest/android/settings/#block-app-when-obscured

The current workaround is as well described in the docs:

> If you want to disable protection, the blue light filter or any app obscuring Cryptomator has to be disabled one time. Afterwards, the settings can be opened and the option “Disable app when obscured” can be disabled. And then the relevant apps can be reenabled again.

We got some feedback by users that even when all apps are disabled on MIUI 12 devices, Cryptomator or better said Android is still of the opinion that an app is obscuring Cryptomator. That is why these users still cannot access the Cryptomator settings to disable the security feature. Which means that Cryptomator cannot be used on these devices (except when the device is rooted and the `preference.xml` is manually adjusted).

During the design of this feature, we decided that all apps that obscures Cryptomator must be deactivated before it is possible to navigate to the Cryptomator settings, as otherwise the user input could be manipulated and this feature could be deactivated on behalf of the user (even without realizing it).

Without these considerations one could simply extend this dialog and for example, directly offer a "deactivate" in it.
**But to reduce the security of the app** only because either the MIUI 12 OS does not show all apps that obscure Cryptomator, or Android on these devices has a bug, **I personally do not accept it...but would like to put it up for discussion**.

TODO: I still need to research if all MIUI 12 devices are affected, other versions too, if this is a known problem and if there are bug reports in Android about this but I wanted to put this on record since some users have already approached us regarding this.

### Motivation

Allow the use of the app despite the "app obscured" dialog if not all apps can be disabled to obscure Cryptomator.