Merge to M72: Activate/deactivate VP9 spatial layers.

* Stop encoding spatial layers S(n >= N) if application deactivates
spatial layer N by setting RTCRtpEncodingParameters.active = false.

* Move calculation of padding bitrate to SvcRateAllocator class.

* Pad up to minimum required bitrate of base layer if ALR probing is
enabled.

Original Reviewed-on: https://webrtc-review.googlesource.com/c/113240

MERGE = approval for merging to M72 obtained in
https://bugs.chromium.org/p/chromium/issues/detail?id=914764

Bug: webrtc:9350
Change-Id: Iefa5d6a4a00bd391c6933ac75383cacf958ca7d5
Reviewed-on: https://webrtc-review.googlesource.com/c/113240
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Cr-Original-Commit-Position: refs/heads/master@{#25945}
Reviewed-on: https://webrtc-review.googlesource.com/c/114423
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/branch-heads/72@{#19}
Cr-Branched-From: 6b3d18164b4ac2eb954663d162644a12eeb13ea6-refs/heads/master@{#25798}