Keep WallpaperPickerGoogleTests in internal cf.

Bug: 301648224
Change-Id: Ia2daf861ad19945d4d15fdd0f43622ad90fca5db
Test: presubmit
diff --git a/TEST_MAPPING b/TEST_MAPPING
index 6345ffb..560897b 100644
--- a/TEST_MAPPING
+++ b/TEST_MAPPING
@@ -2,6 +2,7 @@
   "presubmit": [
     {
       "name": "WallpaperPickerGoogleTests",
+      "keywords": ["internal"],
       "options": [
         {
           "exclude-annotation": "org.junit.Ignore"