In case if primary encoder can't be instantiated (max number of instances has reached, for example), use fallback encoder. Bug: none Change-Id: I477bdeb7af4dcce50e36b1804ffc6ad2ab004dfd Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/234500 Commit-Queue: Sergey Silkin <ssilkin@webrtc.org> Reviewed-by: Niels Moller <nisse@webrtc.org> Reviewed-by: Xavier Lepaul <xalep@webrtc.org> Cr-Commit-Position: refs/heads/main@{#35161}