Show icons for allowed sounds

(Some icons are temporary, until we get the final assets)

Also some fixes/improvements to CircularIconsPreference:
* Show the correct placeholder and +N icons.
* Fix the displayIcons-before-measure case (global layout listener was incorrect).
* Properly cancel pending image load futures (field didn't point to the actual future).
* Don't reload icons if it's the same set (depends on equals() for the items, so unfortunately doesn't work for AppEntry yet).

Test: atest com.android.settings.notification.modes
Bug: 346551087
Flag: android.app.modes_ui
Change-Id: I9d029a5fdd785ada4e2ba4d8a90eba72b5fb9085