Add assignment operator to AudioEncoderRuntimeConfig

Since the copy-constructor is explicitly defined, the coding-style guide mandates explicitly defining the assignment operator, too.

BUG=None
TBR=stefan@webrtc.org

Review-Url: https://codereview.webrtc.org/3014473002
Cr-Original-Commit-Position: refs/heads/master@{#19800}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 3fe3e3b8fdc4539d4c17142dda74536244b52d3f