Don't re-randomize picture_id/tl0_pic_idx when re-initializing internal encoders.

TESTED=video_loopback and AppRTCMobile with forced encoder reinits every 30 frames.
BUG=webrtc:7475

Review-Url: https://codereview.webrtc.org/2833493003
Cr-Original-Commit-Position: refs/heads/master@{#17984}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 080830c513023df6dbf11b42e92c9df59fc8c049
16 files changed