[SB Refactor] Define flags that allow us to run the backend without

updating the UI.

Bug: 238425913
Test: set the _BACKEND flags on and verify that we get the logs but that
the icon comes from the old system
Test: set the _BACKEND flags off and verify that we don't get any logs
from the new system
Test: set the USE_NEW_ICONS flags on and verify that the icon comes from
the new system
Test: statusbar.pipeline tests

Change-Id: I7125da88dbb9da52fc8b22bb9ff34af0c13e290d