metric activation and TTL.

Currently, once a metric config is pushed to statsD, it will
start to collect metrics immediately. This CL introduces the metric
activation logic. When metric needs an activation, the metric producer
will hold until the activation event is detected. Then the metric producer
starts metric generation until the TTL expires (timebomb).

This is to support Mainline where it wants to collect a few metrics for
a few hours when the binary push starts or flag flips.

Test: statsd test
BUG: b/117858835
Change-Id: I992ae98f4303d5b79932eb94eddf6c19ded3727e