Replace taskbar hotseat with real hotseat when folder is open

- Seamlessly show real hotseat and hide taskbar hotseat, while
  keeping rest of taskbar visible
- Update MultiValueAlpha to allow for taking max alpha instead
  of blending, and use that for Hotseat
- Fix folder open bounds on home screen when taskbar is present

Test: Open folder from taskbar on home, can drag out items
Bug: 182079330
Bug: 171917176
Change-Id: I7c1983e3219b1341cf233260f0ccac9051c4dc14