Add importance indicator to conversation icons.

We're also drawing these icons a little differently than the
launcher does, both to accommodate the importance marker, as
well as remove the shadows that are useful for separation
vs. user wallpaper but are not appropriate for the
notification surface or settings screens.

Bug: 149747760
Test: make RunSettingsLibRoboTests
test: atest NotificationConversationInfoTest
Test: manual in notifications UI
Change-Id: I3f73c025b1d32194e3d5147d03cb781e370f2a1b