more notification metrics testing

also add more MetricsAsserts helpers

Bug: 34705522
Test: mmma -j20 platform_testing/tests/functional/notificationtests && adb install -r -g $OUT/data/app/NotificationFunctionalTests/NotificationFunctionalTests.apk && adb shell am instrument -w -e iterations 1 -e class com.android.notification.functional.NotificationInteractionTests com.android.notification.functional/android.support.test.runner.AndroidJUnitRunner && runtest --path platform_testing/libraries/metrics-helper/tests
Change-Id: I57f48f75258d316c256db5adf981ed976387a1bf