Move extern "C" to top of headers and remove tBTA_HF_CLIENT_HDR.

Previously, extern "C" was placed in various locations within the header
files, sometimes below actual struct declarations. Doing so would lead to
alignment issues between C and C++ code.

tBTA_HF_CLIENT_HDR was removed from bta_hf_client_api since it was
extraneous and empty - leading to alignment issues.

Change-Id: Icdd338f1affe4c3a70c7bbd716249be6a16443d2