Refactor MetricActivation initialization

Specifically, MetricActivation state (mEventActivationMap and
mEventDeactivationMap) is now passed into the MetricProducer
constructor. This contrasts with the previous implementation in
which activations were accounted for after the MetricProducer
was constructed.

Additionally, the code to prepare the first bucket has been moved
into the constructor for gauge and value metrics. This last part
resulted in the modification of some tests.

Test: bit statsd_test:*
Test: atest CtsStatsdHostTestCases (no additional tests fail)
Bug: 131240626
Change-Id: I474ee8b1c51b187fbff4520deaf34b27e403bd14