cryptohome: Move Encrypt/DecryptWithSystemSalt() out of CryptohomeLibrary

The implementation in CryptohomeLibrary is moved to CryptohomeTokenEncryptor
as-is, except that CryptohomeLibrary::GetCachedSystemSalt() is called at the
beginning of LoadSystemSaltKey():

  bool CryptohomeTokenEncryptor::LoadSystemSaltKey() {
    if (system_salt_.empty())
      system_salt_ = CryptohomeLibrary::Get()->GetCachedSystemSalt();

BUG=303474, 298605
TEST=none
R=davidroche@chromium.org, hashimoto@chromium.org

Review URL: https://codereview.chromium.org/25975002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227631 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227631