Fix skipped bucket end times

Previously, skipped bucket were assigned the full bucket end time by
default. This caused partial buckets that were skipped to have the
wrong end time. The end time is now logged when the bucket is being
flushed so that the actual bucket end time is captured.

Bug: 148692479
Test: bit statsd_test:*
Change-Id: Ie9cc57d1bac5f6522e5794b4a49b99d1208f44ea