Build System: fix autogen_test_config problem

If there happen to be any dependencies added to target of $(test_config),
it'll cause problem in the autogen_config. We fix it by using $< in
the autogen command.

BUG: 109736180
Test: lunch walleye_coverage-userdebug; make adbd_test
Change-Id: I22ab100c5a3f9fa058c1edd66f9e5cfdf5b7f855
diff --git a/core/autogen_test_config.mk b/core/autogen_test_config.mk
index 20c582a..5feec2b 100644
--- a/core/autogen_test_config.mk
+++ b/core/autogen_test_config.mk
@@ -31,7 +31,7 @@
 # Auto generating test config file for native test
 $(autogen_test_config_file) : $(autogen_test_config_template)
 	@echo "Auto generating test config $(notdir $@)"
-	$(hide) sed 's&{MODULE}&$(PRIVATE_MODULE)&g' $^ > $@
+	$(hide) sed 's&{MODULE}&$(PRIVATE_MODULE)&g' $< > $@
 my_auto_generate_config := true
 else
 # Auto generating test config file for instrumentation test