Simplify logic for connect in HfpClientProfile

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

Bug: 111812003
Test: make -j42 RunSettingsLibRoboTests ROBOTEST_FILTER=HfpClientProfileTest
Change-Id: Ie36efd6d0a9f8e6501a3690b026ea4bd39744b5c
(cherry picked from commit 2ecfa2abe6bee9318dcc5f0b4c3b3d3ec634f0e4)