Keep GtsContentTestCases in internal cf.

Bug: 300519349
Change-Id: I72d27478f970db1bf99725906df779e2d9f179a8
Test: presubmit
diff --git a/services/core/java/com/android/server/pm/TEST_MAPPING b/services/core/java/com/android/server/pm/TEST_MAPPING
index 390c45b..0fd7a37 100644
--- a/services/core/java/com/android/server/pm/TEST_MAPPING
+++ b/services/core/java/com/android/server/pm/TEST_MAPPING
@@ -55,6 +55,7 @@
     },
     {
       "name": "GtsContentTestCases",
+      "keywords": ["internal"],
       "options": [
         {
           "include-filter": "com.google.android.content.gts"