Initial notification blocker

Does not currently track stats across reboots.

Test: make ExtServicesUnitTests &&
adb install -r $OUT/data/app/ExtServicesUnitTests/ExtServicesUnitTests.apk &&
adb shell am instrument -w android.ext.services.tests.unit/android.support.test.runner.AndroidJUnitRunner
Bug: 63095540

Change-Id: Ie3a299cdfb229dedf85a07de5cc19f7a8ea423e0