media/gpu/vaapi: Reduce max simultaneous decoders on Grunt to 10

VaapiVideoDecoder has the notion of having a maximum number of
concurrent decoders, currently 16. For AMD Radeon R5, a.k.a. Stoney
Ridge as those in Chromebook "grunt", this is too high, and should
be no larger than 10.

This CL moves the calculation of said maximum to VaapiWrapper (by
default 16, Grunt 10) and adds a private GetMaxNumDecoderInstances()
for convenience and for the new unit test.  With this, the
Create/ForVideoCodec() will fail (returning a nullptr VaapiWrapper)
when the threshold is reached, and afterwards and while there's too
many active decodes.

The querying point also moves from VaapiVideoDecoder::Create() to
Initialize() since it's at that moment that resources are acquired --
actually they might even be acquired later, and not upon the
VaInitialize() or VaapiWrapper::Initialize() calls in
VaapiWrapper::Create(), but it's certainly an improvement from ToT.

[1] https://source.chromium.org/chromium/chromium/src/+/main:media/mojo/services/mojo_video_decoder_service.cc;l=230-235;drc=581b9e8403fdba9f33cc315c56180f841407c86b

Bug: b:266003084
Change-Id: I51e764a74fab5f81384ea923d1fcb06fc1a6ea2a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5304614
Reviewed-by: Andres Calderon Jaramillo <andrescj@chromium.org>
Commit-Queue: Miguel Casas-Sanchez <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1267371}