Sleep in between calls to NoMan service in test

NotificationManagerService has a non-mockable relationship to
System.currentTimeMillis and this test assumes that calls into the
service will not happen in the same millisecond. The fix here is to
sleep for 1ms in between calls to ensure that the clock advances.

Test: atest --rerun-until-failure 100 NotificationManagerServiceTest
Bug: 155331470
Change-Id: Ica6b26a3d72295b0be52c6b1579dde30447ddeaf