Statsd-log notification channel changes.

Logs when a notification channel is created, substantially modified, or deleted.
Channels are identified by app uid & package, and a small hash of the channel ID.
Distinguishes user modifications from app modifications.
Distinguishes conversation (system-created sub-channel) from regular
channel create/delete.

Bug: 146488473
Test: atest com.android.server.notification.PreferencesHelperTest
Test: adb shell cmd stats print-logs && adb logcat -s statsd:I | grep ' (246)'
Change-Id: Icf2b2707a69dc9d2fb5fa59d0033930e067f201b