Default cmd notification commands to current user.

This makes it possible to run CTS tests against a secondary user. Please
see go/aae-cts-on-u10 for more information.

bug: 121282625
Test: After switching to a secondary user: cts-tradefed run cts-dev -m CtsAppTestCases -t android.app.cts.NotificationManagerTest
Change-Id: I49c8f106bcda0d9b5a8303b9b8088a8989721993