Integrated A2DP LDAC support with the LDAC encoder library

* Add compile-time dependency on the libldacBT_enc module
  in external/libldac
* Use the <ldacBT.h> header and remove local typedefs and defines

Also, fix an initialization issue of the LDAC handler
inside file a2dp_vendor_ldac_encoder.cc

Bug: 30958229
Test: A2DP streaming to LDAC headset
Change-Id: I3757230c9901509daab36f2b2018faa05490171d