Optionally create package preferences in getNotificationChannels()

This behavior is to mimic existing behavior of getNotificationChannel(). In the (rare) case that an app requests a channel but the package preferences haven't been created yet, getNotificationChannel() would create it and the default channel if necessary. This change has getNotificationChannels() do so as well, but only when it's called via NotificationManager.getNotificationChannel.

Bug: 381131846
Test: NotificationManagerTest, PreferencesHelperTest
Flag: android.app.nm_binder_perf_cache_channels
Change-Id: I429ae8ac5e72878380399c81ebe6d5b73becd1ce