Skip invalidating caches in NotificationManagerServiceTest

The IpcDataCaches used in NotificationManager and invalidated in PreferencesHelper cannot be invalidated in test processes unless the caches are set to test mode. Since this test is not intending to test caching behavior, make sure the component PreferencesHelper doesn't try to invalidate real caches when the test is running.

Most tests in NMSTest use the mock PreferencesHelper anyway, but some do go through the "real" PreferencesHelper object created in TestableNotificationManagerService.init().

Flag: android.app.nm_binder_perf_cache_channels
Bug: 381131846
Test: this
Change-Id: I2f1922b2c5e038b256572add3b7d8514e11562c6