base: nest "receiver scheduled" slices under "process running" slices

As noted in b/259535570 and updated documentation for ag/20519869, we
now support nesting async slices. Since "receiver scheduled" slices
perfectly nest until "process running slices", it makes sense to use
the same cookie to allow parent <-> child relationships between
the two.

This does *not* change the UI/UX of traces in the Perfetto UI: it just
makes these slices much easier to query using SQL.

Bug: 259535570
Change-Id: Ie3a0f493ccd3a7f69e946e55486b2991b69c5ee3