Use a temporary summary in the "Apps that can interrupt" preference

We load the list of apps in a background thread, so getting that string later was causing the preference to increase in height, with an annoyingly noticeable animation. Having a placeholder test ensures this doesn't happen (and actually loading is pretty fast, so this text lkely won't even be visible).

Fixes: 347744856
Test: atest ZenModeAppsLinkPreferenceControllerTest
Flag: android.app.modes_ui
Change-Id: I4f9f60104fb8b0abbc7c4279b87daafac481e98a