Remove unused support for "encrypted" boot animations

Since support for Full Disk Encryption was removed in favor of
File Based Encryption, "CryptKeeper mode" no longer exists and the
"vold.decrypt" system property is never set.  Therefore, remove the
unused code that supported showing special boot animations in this mode.

Bug: 208476087
Change-Id: I3fa2549338e1889dd9e9b381c60c264b95181b35