Improve App notification loading

1) Use UiBlocker, for better animations for apps with a lot of channels
2) Only load data in onResume, because the data loading step in some
of the controllers is expensive

Test: make -j64 RunSettingsRoboTests ROBOTEST_FILTER="com.android.settings.notification"
Test: load small channel and large (6000ish) channel app pages
Test: load individual channel pages
Test: load in-app channel shelf UI
Fixes: 215072888
Change-Id: I9a1c96f75b02f94b3ffc529d17d9c0cad7752de1