Add java documentation to VirtualFileSystem and make a cleartext mount explicit
cleanup the old BlockGuard cruft leftover from the AOSP import

BlockGuard was used for threading, but we handle threading in libsqlfs.
Cleanup FileDescriptor class and improve docs
Cleanup IOCipherFileChannel cruft and TODOs
Cleanup File.java TODOs

Most of these are old TODOs that aren't relevant to sqlfs (e.g., the close one) or are not important.
Cleanup TODOs in Os.java
Remove tons of crufty code from IOUtils

It isn't used anywhere, so we'll delete it. The final TODO doesn't seem justified.
Update TODO with our ticket number
Remove unused import