Shave ~150ms off gVisor startup by making host membarrier lazy + async.

[copybara-service]

Shave ~150ms off gVisor startup by making host membarrier lazy + async.

Host membarrier support lookup and registation used to run as part of init
for the hostmm package. Due to runsc being a monolithic binary, this
executes unconditionally several times during the sandbox startup process.

This change skips this process for all runsc invocations that don't need
it by moving it to the Platform constructors, and makes initialization
itself async because the MEMBARRIER_CMD_PRIVATE_EXPEDITED probe can take
tens of milliseconds by itself.

Benchmarks:

# Plain Docker, Systrap in a VM, running just
# `docker run --runtime=runsc --rm alpine true`:
                 │    before    │                after                │
                 │     sec      │     sec      vs base                │
DockerRunBinTrue    836.5m ± 4%   674.6m ± 4%  -19.35% (p=0.000 n=16)


# Kubernetes with containerd, measuring time-to-first-log latency
# minus Kubernetes scheduling overhead:
        │          before          │                      after                       │
        │ totalruntimeinit-min-sec │ totalruntimeinit-min-sec  vs base                │
StartUp               1083.7m ± 2%                943.9m ± 3%  -12.91% (p=0.000 n=16)

        │          before          │                      after                       │

        │          before          │                      after                       │
        │ totalruntimeinit-p50-sec │ totalruntimeinit-p50-sec  vs base                │
StartUp               1083.7m ± 2%                943.9m ± 3%  -12.91% (p=0.000 n=16)

        │          before          │                      after                       │
        │ totalruntimeinit-p95-sec │ totalruntimeinit-p95-sec  vs base                │
StartUp               1083.7m ± 2%                943.9m ± 3%  -12.91% (p=0.000 n=16)