AfW custom lock screen message - API polish

DevicePolicyManager:
* getDeviceOwnerLockScreenInfo now returns CharSequence as it returns a string
  for display to a user
* setDeviceOwnerLockScreenInfo
** accepts CharSequence, not String as this is a string displayed to the user
** Returns void; throws an appropriate runtime exception on failure

Bug: 27531295
Change-Id: I30528569cfa66ee76f857fbee1c3196f821718fd