Deprecate FDE constants from DevicePolicyManager

Deprecate the DevicePolicyManager.ENCRYPTION_STATUS_* constants that can
no longer be returned by
DevicePolicyManager.getStorageEncryptionStatus() due to them being
specific to Full Disk Encryption.

Also update DevicePolicyManagerService.getEncryptionStatusName() to stop
referring to the deprecated constants when it doesn't need to.

This is a follow-up from "Drop FDE status checks from
DevicePolicyManagerService" (https://r.android.com/2052073).

Bug: 208476087
Change-Id: I7110711dfc3d32045ee443dc1ae00e8efb4f8d6e