[Mobile] Add support for KEY_SHOW_5G_SLICE_ICON_BOOL carrier config

This is a carrier config I wasn't aware of when adding the initial
implementation for the network slice indicator. It's a simple boolean
config that defaults to ON.

This CL just changes out the `showSliceAttribution` flow to combine over
the config value along with the `hasPrioritizedNetworkCapabilities` that
we've been using until now.

Test: MobileIconInteractorTest
Test: MobileConnectionRepositoryTest
Fixes: 343766035
Flag: NONE bugfix
Change-Id: I74eb6d47aa0707ddb5f3e3d403fe8f2b48157fef