Allows AccountManager to start exported activities.

We prevent authenticators from using Settings to launch arbitrary
activities by checking if the activity has the same signature with the
authenticators. However, some activities
(GrantCredentialsPermissionActivity and CantAddAccountActivity) are
returned by AccountManagerService and should be allowed to start.

Bug: 18864662
Change-Id: I20c17f0864e6cee1cca901514da2638f551e5423