wear_ui: Remove backgroundIcon.

It's covered by the equivalent variable in ScreenRecoveryUI:
  GRSurface* error_icon;

Also refactor WearRecoveryUI::draw_background_locked() to get it
closer to ScreenRecoveryUI code.

Test: Build a wearable target recovery; Run graphics test.
Change-Id: I3a8d0e4dbf6fe170e4f3adde7eaf4a2043132a57