Change the test mapping configuration of GTS modules by using "imports."

Reason: Test mapping files of GTS modules should use the indirection to prevent
revealing the module names.

Bug: 130788465

Test: atest --test-mapping frameworks/base/api/TEST_MAPPING
Change-Id: I51d969240fd3a3af7740586693fdc1e89dcf451e

api/TEST_MAPPING

@@ -5,9 +5,11 @@
     },
     {
       "name": "CtsSystemApiSignatureTestCases"
-    },
+    }
+  ],
+  "imports": [
     {
-      "name": "GtsUnofficialApisUsageTestCases"
+      "path": "vendor/xts/gts-tests/hostsidetests/api"
     }
   ]
 }