Add `TraceCollector::IsEnabled(bool)` method in order to test when tracing is enabled.

Some builds install a `TraceCollector` at process startup, but it is mostly not enabled. This inhibits the recent optimization to avoid accessing `OpKernel::name()` and `OpKernel::type_string()` every time a kernel is launched. By caching the `TraceCollector` in the `TracingDevice` and adding a method to enquire about its state, we increase the applicability of the optimization.

PiperOrigin-RevId: 211752728