Clear duration when creating exception

When creating a single-instance exception to a recurring event,
we're expecting to have a DTEND but no DURATION.  We were
neglecting to clear DURATION.

Also, don't act as a sync adapter when creating an exception in
the non-sync-adapter test.

Bug 5116044

Change-Id: I16d4e927cb1876a636ec9d6b0c2736df4a25af23
2 files changed