floss: Fill in DBusHeadsetCallbacks for HFP SLC

Add implementation for some AT command callbacks.
 - The internal entry point for callbacks is bta_ag_at_hfp_cback()
 - Some AT commands are considered 'local' and is handled in bta_ag_cmd.cc
   with build time defined values
 - Otherwise by default AT commands are forwarded to application(i.e HFP shim)
   to be handled, and that's what this CL is for.

Bug: 195344366
Tag: #floss
Test: Build and check btmon
BYPASS_LONG_LINES_REASON: Bluetooth likes 120 char lines

Change-Id: I5b5439d5010b558facee2790d4eaede1365ba81d