http2: fix RFC 9218 write scheduler not being idempotent

Previously, the RFC 9218 write scheduler had a bug where AdjustStream()
did not update the stream's metadata after adjusting its priority. This
results in the function not being idempotent, where repeated calls to it
for the same stream can instead remove an unrelated stream from our
scheduler, and duplicate the stream whose priority is being updated.

For go/golang#75500

Change-Id: Iaf3dd819d02839bc6cff65027c4916f9f2fa3e5b
Reviewed-on: https://go-review.googlesource.com/c/net/+/709477
Reviewed-by: Nicholas Husin <husin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>