ffmpeg: update fec patch to match recent ffmpeg

An AVOption has been added to ffmpeg in sept. 2020, making the patch in
contrib redundant. This change is integrated in the contrib ffmpeg
starting from commit 85501986, which updates the used version.

This patch adapts the patches to be applied to ffmpeg and the call to
enable FEC in libopus. Specifically it does the following changes:

- backport the mainline ffmpeg patch that add fec to libopusenc
- add packet_loss reload to adapt its value to network
- modify rules.mak to add patches to the correct platforms
- make media_encoder set the right AVOption to enable fec

Issue: #4857
Change-Id: I614fce2f0cf7fc387973b34323138b07c1767726