Cleanup some taskbar/hotseat handoff issues

- Clearly distinguish Home <-> App icon alignment (ForResumedState and ForGestureState) from Home <-> Home states (ForLauncherState) by renaming onIconAlignmentRatioChanged() to onIconAlignmentRatioChangedForAppAndHomeTransition (for the former)
- Make sure initial state is properly applied by treating all flags as changed
- onIconAlignmentRatioChangedForStateTransition only runs when launcher is resumed (fixes 208648294)
- Animate taskbar background alpha when going between home and app, separate from icon alignment (bug: 204657916)
- Only copy hotseat's alpha to taskbar when hotseat is originally visible (also bug: 204657916)

Test: Open an app on small screen, open large screen and ensure taskbar stays at the bottom instead of aligning with non-present hotseat (bug: 208648294)
Test: Swipe up and down on home scren, ensure taskbar background never appears and there's no jump between taskbar and hotseat
Change-Id: Iea59965118f2bdbd8ce49279f76fb01fbd61f60b