Need to add our Xts Media tests to the test package, so that we can
execute those tests with our nightly regression.
Previous cl (716634) for this was rolled back, as it didn't conditionally
check for the presence of vendor/xts to the build environment before adding
the dependency. This cl corrects that, and verified by removing vendor/xts
from the build environment, and validated that both make tests built
successfully, and the generated google-tests.zip was created as expected
(only missing XtsMediaTestCases.apk).
Bug: 22094009
Change-Id: Ia4e5ee0abbaf1e0efc591d6db34aa3f35ac62330
5aff60980e