Simplify logic for connect in PbapClientProfile

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

Bug: 111812003
Test: make -j42 RunSettingsLibRoboTests ROBOTEST_FILTER=PbapClientProfileTest
Change-Id: Ide43ee5cec560945ad69639782ad6f214ea0cece
(cherry picked from commit 12ed2468d290d5c861e5d1d88a763630dabe97da)