KeygaurdBottomArea additional API cleanup

* Moves alpha setting of the ambient indication area into KBAV, as it
  already manages alpha for other child views
* Removes the need to get the ambient indication area from
  CentralSurfaces
* Cleans up overly poweful getter that gets the indication area from
  KBAV to only return a weaker object: an animator, one per each
  of the two indication areas

Bug: 235403546
Test: manually verified that the bottom buttons and "which song is
playing?" area show, hide, and animate appropriately.

Change-Id: I47760e3d400e01bfb6b1fa1cdaa6ae6dcd5ea521