[SB Refactor] Implement demo mode for carrier merged wifi.

Bug: 238425913
Bug: 264684296
Test: `adb shell am broadcast -a com.android.systemui.demo -e command
network -e wifi carriermerged -e level 2` -> displays a mobile triangle
with level 2 and "W+" as its RAT type
Test: See additional test steps in second bug
Test: existing demo mode commands for wifi and mobile still work
Test: atest DemoModeConnectionsRepositoryTest

Change-Id: I9414c0b2566dcc2c49fe78ad352f0cc981ae6404