Change LocationTimeZoneEvent elapsed realtime

Switch LocationTimeZoneEvent to elapsedRealtimeMillis from nanos. This
makes it more consistent with other usages of the elapsed realtime clock
in time zone detection. This information is currently only used for
debug so there will be no functional change.

Also change LocationTimeZoneEvent toString() to additionally report
mElapsedRealtimeMillis value using Duration.toString(); this is done
elsewhere in debug output for time zone detection so makes understanding
log output easier.

Bug: 169304499
Test: build only
Change-Id: If307c9b27d64ca49a4fcc0d9bb6e36f362990b3e