Usually, mShouldFallbackIfError has been reset to false in DataCallback()
before Stop() is called. However, if fallback to a system clock driver due to
cubeb error had already occurred, then mShouldFallbackIfError would not have
been reset, and Stop() is still called. With mShouldFallbackIfError still
true, a cubeb error in stop would have created another fallback thread.
I expect that resetting mShouldFallbackIfError in Stop() would also be an
effective alternative solution, but resetting on StateCallback() happens
earlier, which would be an advantage if any additional errors could possibly
be reported to StateCallback().
MozReview-Commit-ID: E9j7PQmS3O4
