HFP: Handle network callbacks properly

- As the network callback is initialized at
  two places, it is leading to exception while
  unregisteting the network callback. This is
  due to callback memory is different at
  register and unregister.

- Ensured callback is initialized at one place.

CRs-Fixed: 2435461
Change-Id: Ic3dd3ac977ac442852ebca5c0b51acbe789d8177
1 file changed