sf: only look for graphics frame packets when asserting on traces

Asserting on packet counts leads to extremely brittle tests as it
prevent the Perfetto team from adding new service-generated packets
as needed.

Instead, perform all assertions on packets which have already been
filtered to contain just packets of relevance to sf - namely
GraphicsFrameEvent packets.

Change-Id: I0c323761acd35ca4d3b9e21c2a68d1baefdcd13a