Return copies of notification channels and groups instead of the cached entities.

This fixes an issue where callers could in theory modify the returned object from getNotificationChannel or getNotificationChannelGroup and as a result modify the cache.

Also modifies NotificationChannelGroup.clone() to actually return a deep copy by also copying the list of channels and its contents.

Flag: android.app.nm_binder_perf_cache_channels
Bug: 381131846
Test: NotificationManagerTest, NotificationManagerServiceTest
Change-Id: I950695c8ff89924a998a0b79bfdbacfd27fbe4f0