[FIX] Duplicated call onAnswer. New open method.

[efraespada]

• Allows more flexible workflows for managing phone accounts without alerts.
• Fixes duplicated call of onAnswer.

  // always is called
  void onAnswer(int videoState)
  // and after:
  void onAnswer() <-- now this is ignored

  But the void onAnswer(int videoState) method (in VoiceConnection) stills being invoked twice. Before this change, CallKeepPerformAnswerCallAction was called x4.

• Added support for drawable icons for the foreground service's notification:

  'drawable/ic_notification_launcher'
  'mipmap/ic_notification_launcher'

• Readme updated

[efraespada]

Not merged yet