S3AFileStatus to declare that isEncrypted() is always true

Some bits of yarn treat encrypted files differently than unencrypted ones, and get confused by S3A (and other stores) where the permissions say "world readable" but they aren't really.

Proposed: always declare that the file/dir is encrypted.

Need to fix ITestS3AContractOpen to skip the encryption test, or just push it down to HDFS/webhdfs, as the rule "empty files are not encrypted" doesn't have to hold elsewhere