Use separate AnswerFragment for Talkback

+ Split AnswerFragment into GlowpadAnswerFragment and
  AccessibleAnswerFragment, with the refactored AnswerFragment
  as base class.
+ InCallActivity will select GlowpadAnswerFragment and
  AccessibleAnswerFragment base on if any touch exploration
  accessibility service is active.
+ Removed AnswerListener from GlowPadWrapper, as it is only
  implemented in one class and made the code more complex.
+ Changed answer icon's color into green.

Bug:24101341
Change-Id: Ida2b07986d64d6442ff87a2258180846a092942e