[tracing] Replace magma scheduler async event with a vthread duration

As part of our ongoing effort to better capture GPU work through
tracing, this CL converts the Context Exec async event into a
corresponding vthread duration event.

Test: tiles traces with and without --driver.enable.tracing=1

Change-Id: I8596dc99b6be1c0ad03400fb40ee4cfa91d20f1a
5 files changed