Fix incorrect spatial id handling wrt frame metadata handler.

In some cases when simulcast is used, the spatial id is explicitly set
to 0. In VideoStreamEncoder, this lead to a problem the `stream_idx`
was set to 0 for all simulcast streams - prevent proper handling in the
frame encoder metadata writer.

This can be solved by simply taking the maximum of simulcast and spatial
id (or zero).

Bug: webrtc:42225300
Change-Id: If2b65ad0fdf9a44fb9e1bfb28adf87402b864a20
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/488380
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48181}