service: Support startup tracing via SMBs supplied by producers

Adapts SharedMemoryArbiter to support creation of unbound arbiters and
trace writers for startup tracing, intended for use with
producer-provided shared memory buffers.

While unbound, trace writers can be created and associated with a
startup tracing session using CreateStartupTraceWriter().

Because the target buffer ID for these TraceWriters is unknown until
the tracing session is started by the service, they use a placeholder
buffer ID that is translated by SharedMemoryArbiterImpl once the
startup tracing session's target buffer is bound.

In unbound state, the arbiter will buffer any messages for the service
until the arbiter and the startup session are bound.

Chromium side: https://chromium-review.googlesource.com/c/chromium/src/+/1978682

Bug: 147742905
Change-Id: I64aef618d189360340c0da73b835a71b96eb7904