Remove app icon prototypes code

Now the "real" code is behind notifications_redesign_app_icons, so we
don't need the old prototypes anymore.

I also removed two unused Notification.java fields since I was looking
for unused warnings there to make sure I didn't miss anything.

There's a change in IconManager that I wanted to keep (caching the icon
all the time as opposed to only for conversations), so I updated it to
be done behind the new flag.

Bug: 371174789
Test: builds
Flag: EXEMPT removing flagged code
Flag: android.app.notifications_redesign_app_icons
Change-Id: Icbc05f22ac03ea410e310ec31139bc7902233923