More work on device admins:

- You can now show a dynamic message to the user when asking to
  have your DeviceAdmin added.
- A DeviceAdmin can now provide a warning message that is displayed
  before a user disables it.
- Better ordering (and text) of the policy warnings.
- New API to set the maximum failed password attempts before the device
  wipes itself.
- We now store the number of failed unlock attempts in persistent
  storage.
- New managed dialog APIs that will be used by the settings app.

Also a little bit of cleanup as I was working on this - removed the
long unused MailboxNotAvailableException, fixed a java doc in Messenger.