Enhance SAF with app protection

[tobiasKaminsky]

As discussed with @grote, the current SAF's behaviour is unclear to users.

When selecting app protection we should

• explain that when enabling no SAF is working anylonger
• show if persistent uri access is granted
• have checkbox "delete persistent uri access", if above is true
  • opt-out to keep these, e.g. for seedvault
  • https://developer.android.com/reference/android/content/ContentResolver#getPersistedUriPermissions()

[grote]

To clarify, you would use [ContentResolver#getOutgoingPersistedUriPermissions()](https://developer.android.com/reference/android/content/ContentResolver#getOutgoingPersistedUriPermissions()) to get the permissions that were granted to other apps.

With a quick search, I didn't find any way for how you can revoke them.

[jmue]

From my knowledge I think https://developer.android.com/reference/android/content/Context#revokeUriPermission(android.net.Uri,%20int) is the key here.