Launch channel settings as half sheet

If opening app has requested only a subset of fields

Test: manual - launch from an app, filtered and unfiltered, for
normal channels and conversations
Bug: 177246841
Change-Id: Ifd70478101d1ea1340d2ecc55033fab55e65ca92

Change-Id: I5194b959c82b2cfa7990c84285aaf69464cff3a8

src/com/android/settings/notification/app/HeaderPreferenceController.java

@@ -92,6 +92,7 @@ public class HeaderPreferenceController extends NotificationPreferenceController
                     .setRecyclerView(mFragment.getListView(), mFragment.getSettingsLifecycle())
                     .done(activity, mContext);
             pref.findViewById(R.id.entity_header).setVisibility(View.VISIBLE);
+            pref.findViewById(R.id.entity_header).setBackground(null);
         }
     }