Adding clean variables for some variables used in CTS.

1. A CTS package can specify the test runner used to launch
   its tests from the device. This is useful for preparing
   and cleaning the device through some shell commands if
   necessary.

bug:5946699

Change-Id: Ib1d6454c4274b1c37323235c9b3a195f3b9d8131

core/clear_vars.mk

@@ -117,6 +117,7 @@ LOCAL_NO_CRT:=
 LOCAL_PROPRIETARY_MODULE:=
 LOCAL_MODULE_OWNER:=
 LOCAL_CTS_TEST_PACKAGE:=
+LOCAL_CTS_TEST_RUNNER:=
 
 # Trim MAKEFILE_LIST so that $(call my-dir) doesn't need to
 # iterate over thousands of entries every time.