Teach StartupAnalyzer to handle threads with no slices

Sometimes we end up with threads that have no slices to them, which was crashing
the startup analyzer. This change uses POSITIVE_INFINITY as the start time for
threads with no slices.

This change also includes a regression test that was generated by reducing a
trace that exercised this bug. Further regression tests should now be easier to
add.

Test: atest
Change-Id: Ibc82abb6406689b2e1f8c15558458d16e9f3f65d