Revert^2 "Cache calls to getNotificationChannel*."

This reverts commit d8a32d0c.

Reason for revert: rolling the change forward, but this time without breaking FontScaleConverterActivityTest. Fixed by replacing TestableContext with ContextWrapper

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

Change-Id: Ied7babe1382228890f92ca1440ed13a5d4ea151e