Improve the slow query instrumentation.

On user-debug and eng builds, you can set the
"db.log.slow_query_threshold" system property to queries that
take longer than the specified number of milliseconds.
Set it to 0 to log all queries.

This property has been around for a while but it was implemented
poorly.  In particular, it *changed* the behavior of the query
by calling getCount() while holding the Db connection.
In normal operation, the query will not actually run until later.

By putting the timing logic into fillWindow() instead, we ensure
that we only measure queries that actually ran.  We also capture
cases where the cursor window gets filled multiple times.

Bug: 5520301
Change-Id: I174f5e1ea15831a1d22a36e9a804d7755f230b38