Suppress notif block/silence settings for essential packages.

Companion to change I6b8fa374.

Bug: 31360343
Test: Manual. With config_nonBlockableNotificationPackages
      set to include com.android.dialer, examine Dialer's
      notification settings. Be sure to test with advanced
      notification controls enabled in System UI Tuner.
Change-Id: Ifbe13acf4cb02d6642f5c80e000f5b558f289920