Add content description to the stacked mobile icon

This simply combines both content descriptions. Each already contains the carrier name and the number of bars. The active connection is listed first

Test: manually with demo mode and talkback
Test: StackedMobileIconViewModelTest
Fixes: 412700798
Flag: com.android.settingslib.flags.new_status_bar_icons
Flag: com.android.systemui.status_bar_root_modernization

Change-Id: I74edd9fcf437686e09748306a2b075056d3b9d65