Pass initial conditions to combination conditions and metrics

SimplePredicates have a configurable initial value that can be set to
unknown or false. Previously, CombinationPredicates and MetricProducers
were automatically initialized with an unknown condition value instead
of being synced with the conditions of the SimplePredicates that they
rely on.

An initial condition cache is used to store initial conditions of
ConditionTrackers after they are initialized.
CombinationConditionTrackers evaluate their initial condition based on
the initial conditions of their child SimpleConditionTrackers.
MetricProducers also use the initial condition cache to set its
condition during initialization.

Added unit tests in SimpleConditionTracker_test to check that
SimpleConditionTrackers have the correct initial conditions based on
what the InitialValue is set to and the condition is updated properly
after the first few condition change events.

Added unit test in MetricsManager_test to check that all
ConditionTrackers and MetricProducers have the correct initial
conditions.

Test: m statsd_test && adb sync data && adb shell
data/nativetest64/statsd_test/statsd_test64
Bug: b/156762672

Change-Id: I9f6088f8c92fb18eb2ca8632aaa338fb0ed8e679