Mark TelecomManager#isRinging as a TestApi

Bug: 150320750
Test: atest BackgroundCallAudioTest
Change-Id: I048960feb97cd6cf081056ee649e7ea8dacdc8e3
Merged-In: I048960feb97cd6cf081056ee649e7ea8dacdc8e3
diff --git a/telecomm/java/android/telecom/TelecomManager.java b/telecomm/java/android/telecom/TelecomManager.java
index ce71511..b3bf507 100644
--- a/telecomm/java/android/telecom/TelecomManager.java
+++ b/telecomm/java/android/telecom/TelecomManager.java
@@ -1630,6 +1630,7 @@
      * @hide
      */
     @SystemApi
+    @TestApi
     @RequiresPermission(anyOf = {
             READ_PRIVILEGED_PHONE_STATE,
             android.Manifest.permission.READ_PHONE_STATE