Update setOrganizationColor javadoc

Background
* In Android R, the UI of the work challenge was
  updated to latest material spec which no longer
  uses the organization color.

Changes
* Update javadoc of setOrganizationColor mentioning
  the organization color will no longer affect the
  work challenge starting from R.

Bug: 153607870
Bug: 153607569
Test: verify no build breakages
Change-Id: I071f544e4fba1915d810fa741dbc79966f9e72f4