Fix test_http_server location.

The test_http_server is not a gtest executable, but a test helper used
by the unittests. It is expected to be in the same directory.

Bug: 26955860
TEST=Ran HttpFetcher unittests.

Change-Id: I8d05212089d6c774c59c85dc8299984757bbb29d
diff --git a/Android.mk b/Android.mk
index 26d6001..d529f70 100644
--- a/Android.mk
+++ b/Android.mk
@@ -722,6 +722,7 @@
 ifdef BRILLO
   LOCAL_MODULE_TAGS := eng
 endif
+LOCAL_MODULE_PATH := $(TARGET_OUT_DATA_NATIVE_TESTS)/update_engine_unittests
 LOCAL_MODULE_CLASS := EXECUTABLES
 LOCAL_CPP_EXTENSION := .cc
 LOCAL_CLANG := true
@@ -733,7 +734,7 @@
 LOCAL_SRC_FILES := \
     common/http_common.cc \
     test_http_server.cc
-include $(BUILD_NATIVE_TEST)
+include $(BUILD_EXECUTABLE)
 
 # update_engine_unittests (type: executable)
 # ========================================================