Log widget memory stats to statsd

This change adds logging to AppWidgetServiceImpl for the
WIDGET_MEMORY_STATS atom. This atom records the current bitmap memory
usage of widgets held in system server.

Test: statsd_testdrive 10234
Bug: 391574081
Bug: 377356756
Flag: EXEMPT adding metrics
Change-Id: Ie4cd2b547077094c4954281a864f9fe25f2ed2be