Priority conversations onbaording screen

Cherry-picked and squashed 3 commits:

Show an onboarding screen for priority conversations with change ids:
    - I5280cff71591f8551016b6ba00d3a579507367cb
    - I0e3d4b1c64cba2cab581abca4ce9377a7c4676de
    - I8ce4373b563ca307e2892ffa35047d95f04a705d

Bug: 151843296
Bug: 151843296
Fixes: 154332812
Fixes: 154638737
Test: atest SystemUTests
Change-Id: I1b1437d25dced586db37e2f0961470e3d5890185