Disable IMA SSAI DASH live streams for now

#minor-release

PiperOrigin-RevId: 428761508

extensions/ima/src/main/java/com/google/android/exoplayer2/ext/ima/ImaServerSideAdInsertionUriBuilder.java

@@ -366,6 +366,10 @@ public final class ImaServerSideAdInsertionUriBuilder {
     if (streamActivityMonitorId != null) {
       streamRequest.setStreamActivityMonitorId(streamActivityMonitorId);
     }
+    checkState(
+        streamRequest.getFormat() != StreamFormat.DASH
+            || TextUtils.isEmpty(streamRequest.getAssetKey()),
+        "DASH live streams are not supported yet.");
     return streamRequest;
   }
 }