Discourage usage of "Do not ask again"

[shagedorn]

There is currently no way to reset a permission's state (nor can the user go to settings and change it there) when a user has once selected "Do not ask again". This option should generally be discouraged:

• Add note in README and/or header docs
• Show alert in sample app to explain the behaviour
• Add "No" option (that triggers the system alert) to sample app

We should also think of allowing to reset this state via public API.