Handle media transcoding enqueue failures

If the media transcoding service is unavailable, enqueing transcoding
requests will fail. Because of the early failure, we wouldn't have
created a transcoding session. This caused an NPE.

Now, we handle null transcoding sessions and log metrics as failed in
those cases

Bug: 188740213
Test: Manual
Change-Id: I5c115a1636a4b10815e323ccaf804f3fbe1c0637
1 file changed