Renamed the aptX and aptX-HD shared libraries

* The names of the expected aptX and aptX-HD shared libraries are
  updated to reflect better their purpose:
  - aptX: libaptX.so -> libaptX_encoder.so
  - aptX-HD: libaptXHD.so -> libaptXHD_encoder.so

Also:
* Add a missing upcall with the codec info the first time the codec
  is selected internally
* Minor cleanup

Bug: 30958229
Test: A2DP streaming to aptX/aptX-HD headsets
Change-Id: Ib2bfc83823190fb7a057353eb47cd71c5b4d5a3a