bta_ag_sco: Init and start HFP SW encode/decode

Add HFP encode/decode interface if property
bluetooth.hfp.software_datapath.enabled is set.

Bug: 349290628
Bug: 315234036
Test: m com.google.android.btservices
Flag: com::android::bluetooth::flags::is_sco_managed_by_audio and HFP
and bluetooth.hfp.software_datapath.enabled

Change-Id: Idd0cd9aacc9fc121f21c2160f41c69c64298a530