Bug:  fxbug.dev/75281

This functionality is added in issue #41523.  Allows runners to request
the singleton instance of `sys::ComponentContext`, while it remains also
accessible to component-specific code that does not have direct access
to top-level objects.

Fixes #41523

Since the trace provider hasn't been created yet, it is not possible to
trace the creation of a trace provider using a TRACE_DURATION event (the
trace enabled and category check will always fail).  While this isn't
causing any urgent problems, remove it to set a good example for other
clients of tracing that might happen to reference this code.

Revert "Migrate flutter_runner from flutter_runner::{Thread,Loop} to fml::{Thread,MessageLoop} (#15118)" (#16277)

This reverts commit 41e8ed07.

Revert "Migrate flutter_runner from flutter_runner::{Thread,Loop} to fml::{Thread,MessageLoop} (#15118)" (#15903)

This reverts commit a50f1ef5.

Migrates the Fuchsia Flutter and Dart runners off the internal tracing
APIs and onto the public SDK.

PT-63

Change-Id: I9acd5aeba5f82e6d58ec324f0f64bf109a413410

Ported from Topaz tree.

This makes room for the Dart runner at //flutter/shell/platform/fuchsia/dart.

Build rules and include paths have been updated as necessary.
No other functional change.