Move the Android test README under the androidTest directory (#9031)

And opt-in to compile without errors

okhttp-brotli/src/test/java/okhttp3/brotli/BrotliInterceptorTest.kt

@@ -13,6 +13,10 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+@file:Suppress(
+  "INVISIBLE_REFERENCE",
+)
+
 package okhttp3.brotli
 
 import assertk.assertThat