Add TEST_MAPPING to system/bt

* Add "postsubmit" test group to TEST_MAPPING in system/bt
* Developer can run tests mannually in system/bt via:
  system/bt$ atest :postsubmit
* Remove legacy configuration files spread across system/bt
* Created unified configuration file system/bt/AndroidTestTemplate.xml
  and add it to the default build config so that developers don't have
  to configure it

Bug: 113359343
Test: atest :postsubmit
Change-Id: I0fdd55b47ef176b5c4ce3d2cd217ffa67cf9bc23