Change SimplePredicate InitialValue default to UNKNOWN

Previous default for InitialValue was FALSE. Currently, no predicates
configure this field.

Added unit tests to test the initial condition changes in Value and
Count metrics.
- ValueMetricE2eTest.TestInitialConditionChanges
- CountMetricE2eTest.TestInitialConditionChanges

Bug: b/156541904
Test: m statsd_test && adb sync data && adb shell data/nativetest/statsd_test/statsd_test
Change-Id: I242bcb0b60455355e699636aae7e620fabc310ef