Simplify logic for connect in A2dpSinkProfile

* connect: remove connected device check logic. A2dpSinkService
  will check it.
* Remove member variable V and related checks.
* wrap if/else statement in curly brackets.
* add robotest for A2dpSinkProfile.

Bug: 111812003
Test: make -j42 RunSettingsLibRoboTests ROBOTEST_FILTER=A2dpSinkProfileTest
Change-Id: Ia0a118d73e5bb35ed3b2b75e67aeb98ae936981f
(cherry picked from commit b31a745fc0264a622ea9565f2d3da9c267f7f346)