Android.mk: make xtest installed in the vendor partition

so that we could make this work with Treble enabled builds too

Signed-off-by: Victor Chong <victor.chong@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
diff --git a/hello_world/Android.mk b/hello_world/Android.mk
index ff7e7c1..194cf66 100644
--- a/hello_world/Android.mk
+++ b/hello_world/Android.mk
@@ -14,6 +14,7 @@
 
 LOCAL_SHARED_LIBRARIES := libteec
 LOCAL_MODULE := optee_example_hello_world
+LOCAL_VENDOR_MODULE := true
 LOCAL_MODULE_TAGS := optional
 include $(BUILD_EXECUTABLE)