Allow AssetStreamAdaptor to be seekable

Bug: 78866720
Test: Manual

It already supports rewinding, and the underlying Asset supports
seeking. Allow it to be seeked so that SkGifCodec can read directly from
it rather than copying data it needs for internal use.

Change-Id: I0765dcf4a507724878a5a700a770d35802c4b7be
2 files changed