[SB Refactor] Add content descriptions for the mobile triangle.

This mirrors `SignalController#getContentDescription`, which is called
from `MobileSignalController#notifyListeners`.

Bug: 238425913
Test: manual: Verified content description updates correctly using demo
mode
Test: atest MobileIconViewModelTest

Change-Id: I2f1f603fd6b0d1257d90fb6addb1403847438415