ConnectivityMetricsLogger: Switch to "pull" model

If subscriber is in external app, its process can be killed. This would
lead to loss of events. To avoid that, we will be storing events in the
logger service. Clients can periodically pull data from it.

Another option for client apps is to register PendingIntent which will be
sent when certain number of new events has been received.

If events are logged at unexpectedly high rate from particular component,
then throttling mechanism will activate and new events will be blocked for
some time from that component.

Change-Id: I710409626eb5a42bcd3fe8126eac6b756fa1ab8b