Add method to return all packages with any channels.

Used for Settings, where before this (and attached) change was generating the list of apps with any channels by calling getNumNotificationChannelsForPackage for each app in sequence. This reduces many binder calls to one per user.

Bug: 394614704
Test: PreferencesHelperTest
Flag: android.app.nm_binder_perf_get_apps_with_channels
Change-Id: Ieb40e378682a9f2950c4a3eb6c2a8637323e82b6