Assign hf client handle from BTA_HF_CLIENT_OPEN_EVT

Bug: 263323082
Bug: 242154811
Test: When Phone and Watch are connected, from Phone side perform
      Bluetooth off/on and disconnect/connect Handsfree profile
      multiple times, make sure it always connects.
Change-Id: I251739b37b0785b5e5abebd4e3d34eb01dfb70c6
(cherry picked from commit ef87eff35756714c656cdcdc9720dcdf351a36a7)