Inject test adaptation resources at construction

InjectAdaptationConstraint() blocked the worker thread on a task
posted to the encoder queue, and had an early return that never set
the event it was waited on, so a stopped encoder would have hung the
caller forever. Both it and InjectAdaptationResource() existed only
for VideoStreamEncoderTest, which calls them from its constructor.

Pass the resources and constraints to the constructor instead and
register them in the init task it already posts. Nothing blocks, and
the ordering is stronger than before: the injections used to race
that task and worked only because both landed on the same queue in
order.

Bug: webrtc:42222804
Change-Id: I112df0d254dff6328409b9be71bc92e12a8fa7bf
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/504421
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48662}