Master pincode implementation

Changed pincode implementation from separate pincode for each login to master implementation, matching iOS and Android.
Changed account storage to use PasswordVault instead of encrypted LocalSettings.
Bugfixes.
Each account has their own mobile policy, which is stored as part of the data that goes with the account. 

The "master" policy is stored separately and compared against the policy of the user that becomes the "active" user.  If the active user has a more restrictive policy the master is updated to reflect it.