Set correct debug flag in test PersistAdSelectionResultRunnerTest

Flags#getConsentNotificationDebugMode is a debug flag. Since this a
system property, the value returned by the default method can be
different across different instances of Flag object.

Overriding the method in the test Flags will make sure that the flag value is
the set correctly.

Fix: 366419855
Test: atest PersistAdSelectionResultRunnerTest
Flag: TEST_ONLY
Change-Id: I35c5ed4708ef9431e79c50e4006ef2c88b9084fd