TranscodingService: Enable java generation for transcoding aidl

This is used for the Java api to talk to service directly.

Bug: 145628554
Test: Build
Change-Id: Ifb4e4c44de8dbe3844660627385dd337d3ccb22a
diff --git a/media/libmediatranscoding/Android.bp b/media/libmediatranscoding/Android.bp
index 2753db4..a2c1af7 100644
--- a/media/libmediatranscoding/Android.bp
+++ b/media/libmediatranscoding/Android.bp
@@ -31,6 +31,12 @@
         "aidl/android/media/TranscodingRequestParcel.aidl",
         "aidl/android/media/TranscodingResultParcel.aidl",
     ],
+    backend:
+    {
+        java: {
+            enabled: true,
+        },
+    },
 }
 
 cc_library_shared {