make MetricsAsserts useful for unit tests

A unit test may collect logs from a fake or a mock.
Those logs can be passed into the assert instead of
a native reader used by functional tests.

Bug: 35138327
Test: mmma -j20 platform_testing/tests/functional/notificationtests && adb install -r -g /mnt/fastroid/master/out/target/product/sailfish/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: I1263aae0b111d6db50d8d43f80a4d3dda166df69