metrics_daemon will export _metrics._AnalyticsReportingState containing
"enabled"/"disabled" depending on the current state and two weave
commands (_metrics._{enable,disable}AnalyticsReporting) to update it.
BUG: 24386281
TEST: manual. Enabled and disabled metrics reporting.
`metrics_client -c` reports the correct answer.
Change-Id: Ic4a5ffc1e9f9cbc0b47ee34d3af83584d95da155