AAC-ELD: support PLC and multiple frames per RTP packet

and fix uninit to free the audio converter.

diff --git a/mediastreamer2 b/mediastreamer2
index 3f2672fcf8a8d8f951047844d38dca1631999d7e..6e4adda46186fdb197a6bca1dd7097b313fff09d 160000 (submodule)
--- a/mediastreamer2
+++ b/mediastreamer2
@@ -1 +1 @@
-Subproject commit 3f2672fcf8a8d8f951047844d38dca1631999d7e
+Subproject commit 6e4adda46186fdb197a6bca1dd7097b313fff09d