Add VideoStreamEncoder tests for DegredationPreference switching.
Add VideoStreamEncoder tests for DegredationPreference switching.

This test ensures that when changing degradation preference,
a resource that was previously downgraded in a different degradation
preference can not adapt up.

Bug: webrtc:11522, webrtc:11523
Change-Id: Id362530408db4c49b0d0b2516be9a11ccc7c8f37
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/175012
Reviewed-by: Ilya Nikolaevskiy <76788ac3efd229e9d156135d9ceb7fef65c45da4@webrtc.org>
Commit-Queue: Evan Shrubsole <7a4ae6b96648f94a29a5ec4b2747fe0ea094fcde@google.com>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#31250}

TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc