Initial implementation for dual sim icon

This adds the stacked dual sim icon when there's exactly two sims.
This includes the RAT indicator but is missing the exclamation mark icon

Bug: 394635586
Flag: com.android.systemui.status_bar_root_modernization
Flag: com.android.settingslib.flags.new_status_bar_icons
Test: manually -- using demo mode
Change-Id: I8bbbaccb29d3736d5f370fe24f754306915ece3a