Cache calls to getNotificationChannel*.

This caches all calls to getNotificationChannel, getNotificationChannel (for conversations), and getNotificationChannels using the resulting list of getNotificationChannels for each package and doing lookups within that list for individual channels.

Bug: 362981561
Flag: android.app.nm_binder_perf_cache_channels
Test: NotificationManagerTest (CTS), NotificationManagerTest (unit), PreferencesHelperTest
Change-Id: Ief1c9390cf551af87a42f348ecb380d36e500597