Add @hide to RttAudioMode IntDef

IntDefs apparently require an @hide annotation to avoid breaking CTS
tests. Adding it here.

Change-Id: Ifd748b55e68091132571df7e74fdb797bd6a83fc
Test: CTS
Fix: 35808951
diff --git a/telecomm/java/android/telecom/Call.java b/telecomm/java/android/telecom/Call.java
index d67a0d6..1507082 100644
--- a/telecomm/java/android/telecom/Call.java
+++ b/telecomm/java/android/telecom/Call.java
@@ -871,6 +871,7 @@
      * party, if it is active.
      */
     public static final class RttCall {
+        /** @hide */
         @Retention(RetentionPolicy.SOURCE)
         @IntDef({RTT_MODE_INVALID, RTT_MODE_FULL, RTT_MODE_HCO, RTT_MODE_VCO})
         public @interface RttAudioMode {}