transcoding: use __ANDROID_API_FUTURE__ in __builtin_available check

__builtin_available 31 fails because we're not officially 31 yet.
Use __ANDROID_API_FUTURE__ for now.

bug: 178144708
test: builds; transcoding unit tests; manually test transcoding works.
Change-Id: I4ccf05f3858feb9e22eb294b4443edc830737204
diff --git a/services/mediatranscoding/MediaTranscodingService.cpp b/services/mediatranscoding/MediaTranscodingService.cpp
index 74477a8..b94dd01 100644
--- a/services/mediatranscoding/MediaTranscodingService.cpp
+++ b/services/mediatranscoding/MediaTranscodingService.cpp
@@ -62,7 +62,7 @@
 
     uid_t callingUid = AIBinder_getCallingUid();
     pid_t callingPid = AIBinder_getCallingPid();
-    if (__builtin_available(android 31, *)) {
+    if (__builtin_available(android __TRANSCODING_MIN_API__, *)) {
         int32_t permissionResult;
         if (APermissionManager_checkPermission("android.permission.DUMP", callingPid, callingUid,
                                                &permissionResult) != PERMISSION_MANAGER_STATUS_OK ||