Rate-limit (presumably) unnecessary NotificationManager.cancel() calls

Also fixed the enqueue rate calcualtion to clarify that we're only interested in the update rate, and added some logging to Slog.

Bug: 384467103
Test: atest NotificationManagerTest + CTS
Flag: android.app.nm_binder_perf_throttle_notify
Change-Id: I2780229b95108e9b802b33a3d7f606017ea27b62