[mobile] fix test if new icons is enabled

The test doesn't care to set one or the other, it's just testing that a
background exists basically. So we can parameterize it based on the
existing on-device flag setting.

Test: MobileIconViewModelTest
Flag: com.android.settingslib.flags.new_status_bar_icons
Bug: 391606042
Change-Id: I281df0c39affc33db48269dc6ca32df16b24e28e