Fix DropBoxManager.Entry parcels with fds.

When the socketpair communication channel was added to
ParcelFileDescriptor, the CREATOR method for DropBoxManager.Entry was
not updated to read the extra fd from the Parcel. This was causing reads
to end too early.

Also removed comments referring to the now-deleted native methods in
the native Parcel implementation.

Change-Id: I49a9691da6ea927382ceb2fcbfdc7985a601111b