Fix dot/icon overlap in notification shelf

This bug was caused by an off-by-one error where
	isOverflowing = iconX > overflowX
should have been
	isOverflowing = iconX >= overflowX

As a result, the icon that should have been hidden
by the dot was considered "not overflowing"
(iconX=overflowX) so it remained visible.

This change corrects the definition of isOverflowing;
refactors isOverflowing and forceOverflow out of
calculateIconXTranslations; adds tests.

Bug: 232056690
Test: NotificationIconContainer

Test: add notifs to short shelf until overflow
      swipe notifs away until short shelf empty
      (no dot/icon overlap)

Test: swipe down on ls notif then let go
      swipe down on ls notif to go to full shade
      add notifs/swipe away notifs in full shade
      scroll in full shade with long shelf
      (no regressions)

Change-Id: Ie270c55f6c9d27b5d0fee73c22fedb5335760179