Use the correct method to check if device is encrypted

DPM's method will return false if encrypted by default password,
preventing the changing of encryption password to lockscreen password.

Check if the device is encrypted by some means, instead.

Also fix a SecurityException when Device Admin queries encryption state
(recent regression)

Bug: 17881324
Change-Id: Id897e61c5e254ab3f8dc569285428a73005303ea