Better handle the Host APEX case in `build/apex/art_apex_test.py`.

Assume a Host APEX has a Debug flavor by default, and abort if another
flavor (Release or Testing) is explicitly specified.

Test: art/build/apex/runtests.sh
Test: art/build/apex/runtests.sh -t (was broken before this change)
Bug: 142938159
Bug: 139277987
Change-Id: Ifcbc07f9e81adbd394242b0c80f09a590db22554
1 file changed