HFP: Fix static check errors

* In bta/ag and btif/btif_hf.cc
* Simplify boolean values
* Use nullptr intead of NULL
* Use C++ style include headers instead of C style ones
* Remove unused struct definitions
* Remove unused code from bta_ag_ci.h/cc
* Use range based for-loop when it can be converted automatically
* Use C++ style zero-initialization instead of memset so that non-POD
  members of structs can be initialized properly
* Use "" to initliaze char array since the rest of the array will be
  zero padded if the array is assigned to a shorter string than its
  length

Bug: 68340193
Test: make, connect to HFP enabled device
Change-Id: I25c1e48ca1cb40629b9b60243ec462f498d8fc24