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/media/libmediatranscoding/TranscodingClientManager.cpp b/media/libmediatranscoding/TranscodingClientManager.cpp
index 09afb1f..87c984e 100644
--- a/media/libmediatranscoding/TranscodingClientManager.cpp
+++ b/media/libmediatranscoding/TranscodingClientManager.cpp
@@ -302,7 +302,7 @@
     }
 
     int32_t result;
-    if (__builtin_available(android 31, *)) {
+    if (__builtin_available(android __TRANSCODING_MIN_API__, *)) {
         if (APermissionManager_checkPermission("android.permission.WRITE_MEDIA_STORAGE", pid, uid,
                                                &result) == PERMISSION_MANAGER_STATUS_OK &&
             result == PERMISSION_MANAGER_PERMISSION_GRANTED) {