Uninstaller: Check if allowed early

Run check for whether the user can request an uninstallation in
UninstallerActivity. This means that this now runs before the user
confirms to uninstall a package. This adds a new failure dialog in
UserIsNotAllowedDialogFragment.

Bonus: move dialog buttons to the right (neutral -> positive) where they
       belong according to material design spec.

Test: Simulated a failure and saw UI.
Change-Id: Id3e3ade62d5422d0dc2cbda0ab91da0d7f5afed1