[Notification] Fix NotificationManagerServiceTest

NotificationManagerServiceTest fails (10 failures) when launched
with different user as 0 (e.g. Automotive platform).
Test could be launched with "--user 0" to fix these test or test
can be adapted not to hardcode user id launching these test.

This CL makes the test adapative to the user id launching unit test.

Bug: 178032672

Test: adb shell am instrument -w -e class com.android.server.notification.NotificationManagerServiceTest com.android.frameworks.tests.uiservices

Signed-off-by: Francois Gaffie <francois.gaffie@renault.com>
Change-Id: I1f124deafd50d257683667ef986b88df1e4b8a75