Fix test failures when NICRefactor flag is enabled

Failures are caused by running view-binder logic inside of unrelated
unit tests; these tests do not actually verify anything pertaining to
the NotificationIconContainer, so this CL restructures the code so that
we can mock out the view-binder.

Actual view-binder logic is verified via existing NIC screenshot tests.

Flag: ACONFIG com.android.systemui.notifications_icon_container_refactor DEVELOPMENT
Test: atest SystemUITests
Bug: 278765923
Change-Id: I1af34b9e96616c5a923d87fbf25625fd36024544