Add @TestApi annotations to CS-side RTT APIs for CTS

CTS tests need both the in-call and connection service sides of the APIs
to test Telecom functionality properly, so annotating the hidden-for-now
connection service APIs with TestApi.

Test: part of CTS
Change-Id: I3711729d7e8c8aff2735f4da9fbd04bcca6b4942
diff --git a/telecomm/java/android/telecom/Call.java b/telecomm/java/android/telecom/Call.java
index 27f7172..d79a102 100644
--- a/telecomm/java/android/telecom/Call.java
+++ b/telecomm/java/android/telecom/Call.java
@@ -981,15 +981,24 @@
          * @return A string containing text sent by the remote user, or {@code null} if the
          * conversation has been terminated or if there was an error while reading.
          */
-        public String read() {
-            try {
-                int numRead = mReceiveStream.read(mReadBuffer, 0, READ_BUFFER_SIZE);
-                if (numRead < 0) {
-                    return null;
-                }
-                return new String(mReadBuffer, 0, numRead);
-            } catch (IOException e) {
-                Log.w(this, "Exception encountered when reading from InputStreamReader: %s", e);
+        public String read() throws IOException {
+            int numRead = mReceiveStream.read(mReadBuffer, 0, READ_BUFFER_SIZE);
+            if (numRead < 0) {
+                return null;
+            }
+            return new String(mReadBuffer, 0, numRead);
+        }
+
+        /**
+         * Non-blocking version of {@link #read()}. Returns {@code null} if there is nothing to
+         * be read.
+         * @return A string containing text entered by the user, or {@code null} if the user has
+         * not entered any new text yet.
+         */
+        public String readImmediately() throws IOException {
+            if (mReceiveStream.ready()) {
+                return read();
+            } else {
                 return null;
             }
         }