Remove scaffolding for multiple dots

Convert int mNumDots => boolean mIsShowingOverflowDot
Combine hasOverflow and hasMaxNumDot => areIconsOverflowing
Remove unused getNoOverflowExtraPadding, getMaxOverflowStart

Bug: 255534964
Test: open shade from lockscreen/launcher
   have enough notifications to show overflow dot in
   notification shelf, swipe away notifications so dot hides
   when shelf icons stop overflowing => no regressions
Change-Id: I26bb4679ea75e1a25ac3b4865e859a3e1941106d