Export libtextclassifier and make some native tests working again

The export script is updated, so that

1. "android-ready" native tests are now exported to Android.
2. Some dead code are removed
3. test_data is removed for now. Our tests that load a test file
   in runtime have never worked on Android. Will bring them back
   once I get them working.

Test: atest TextClassifierService
Test: atest libtextclassifier_tests

BUG: 157134682

Change-Id: I98b25793e9644bee43f416255911d969b31d91b7
diff --git a/TEST_MAPPING b/TEST_MAPPING
index 2b02610..3c8e10b 100644
--- a/TEST_MAPPING
+++ b/TEST_MAPPING
@@ -7,6 +7,9 @@
           "exclude-annotation": "androidx.test.filters.FlakyTest"
         }
       ]
+    },
+    {
+      "name": "libtextclassifier_tests"
     }
   ]
 }
\ No newline at end of file