Use SkFILEStream directly when possible

When decoding from a file descriptor, if the descriptor is seekable and
has no offset, use SkFILEStream directly instead of wrapping in an
SkFrontBufferedStream.
This will let the SkRawCodec take advantage of a seekable stream.
One can see 2x speed up for RAW cases.

Bug: 27097104
Change-Id: I369fbb3af1170c94ec7cd9bb35e8e0007dde9fd5