[Sb] Define a network slice UI for mobile icons

Based on the new flag NEW_NETWORK_SLICE_UI, we start collecting on
the value of the prioritized network information from the mobile
connection repository, and define a new background layer for the mobile
network type view.

Also reworked the mobile network type icon ImageView to be a FrameLayout
with a background layer. This change does add a small amount of extra
padding, which ensures that the actual slice background has enough space
to draw and doesn't cause the icon to widen when it is shown. We might
want to change this in the future to optimize more space away.

Test: MobileIconViewModelTest, ModernStatusBarMobileViewTest
Test: all sysui tests
Test: use the new `slice` field supported by demo mode
Bug: 270385675
Change-Id: I15be2651e0ceb7d053ce4513e58c207935769963