add libandroidfw_tests back to TEST_MAPPING

libandroidfw_test is not triggered when ag/15987448 modify
libandroidfw.  After the experiments in b/180720568,
`cc_test { support_host: true}` should add back to TEST_MAPPING.

Test: atest --dry-run -p frameworks/base/libs/androidfw

Bug: 180720568
Bug: 178646865
Change-Id: I5c97c860670d57504f97ef10ad296fb57176fd5c

libs/androidfw/TEST_MAPPING

@@ -2,6 +2,9 @@
   "presubmit": [
     {
       "name": "CtsResourcesLoaderTests"
+    },
+    {
+      "name": "libandroidfw_tests"
     }
   ]
 }