Adding important people avatars to AoD

Icons will show up on aod, status bar and expanded view. Icons are
currently updated on notification redaction.

We attempt to retreive people icons across a number of sources (in order
of priority):
1. ShortcutManager
2. Notification.getLargeIcon
3. Person.getIcon() from MESSAGE_EXTRAS
4. Notification.getSmallIcon (fallback if everythign fails fails)

Icons are cached when they the notification is an important conversation
and is sensitive

Bug: 144874876
Test: manually on device
Change-Id: I3292d2fbd90e39c4d2fe9eaa5b2855b7b65d89f9