Add process start metrics to AppStartupHelper.

Statsd reports only cold launch metrics for now hence
the test is written only for cold app launches.

Bug: b/123995966

Test: atest CollectorsHelperTest:com.android.helpers.AppStartupHelperTest

testSingleColdLaunchMetric
testMultipleColdLaunchMetric
testDifferentAppColdLaunchMetric

PASS

Change-Id: I9963279c5a78a16c31897eb00d003a7aad857553