Revert deprecation of DevicePolicyManager FDE constants

The API council doesn't want these constants to be deprecated.  This is
a partial revert of http://ag/18171342, which keeps and revises the text
that was added to the documentation.

Bug: 208476087
Bug: 232310299
Change-Id: Ia5112416bcccf4dac2fa8f17ebe39dab0a8177f3