The aom_codec_stream_info struct had an `sz` field. It looks like
this was intended to implement polymorphism, where individual
codecs under the aom API could extend the struct with additional
members while maintaining common fields like frame size.

However, no codec-specific initializer was ever provided,
requiring callers to manually set this field to the expected
value. The extension mechanism isn't used by the av1 codec,
and also wasn't used by the vp8 or vp9 codecs in the libvpx
implementation.

Since it's not used, this commit removes the layer of indirection,
removing the confusing initialization burden and using
aom_codec_stream_info_t directly within the av1 decoder.

Change-Id: I4372a286b1d03518345c83b01c2f51db6cefe211