Add native Android tracing from the engine.

Today, tracing from the engine relies on proxying traces to Dart VMs tracing infrastructure. This works well in the general case but is missing features like always-on trace flows directed towards ATrace on Android. This would have come in really handy when tracing transaction completions on Android.

We should add a trace API that writes the trace directly (without copies) to the `/sys/kernel/debug/tracing/trace_marker` file for always-on traces on Android. These can be used sparingly in the Android subsystem.