ui: don't draw flows for annotation slices
ui: don't draw flows for annotation slices

Annotation slices don't have any flows so trying to draw flows with
them causes query failures in trace processor.

Change-Id: I36be0dae317be772085193d4ee14f4c8c67dd300
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto