Build test APEXes properly.

- Build them with apex_test, not apex
- If the whole module is disabled because KMI version is not in
BOARD_KERNEL_MODULE_INTERFACE_VERSIONS, still build a stub file
so that it can be used as test data.

Test: builds
Bug: 161563386
Change-Id: Iae53a68e17e4e922503bf27096d76ad96640dd99
1 file changed