Tests for AndroidTraceLayer (`tracing_android_trace`)

[DJMcNab]

The sync layer has some nasty code to manage the stack. E.g. (up-to-date at the time of writing):

android_trace/tracing_android_trace/src/sync_layer.rs

Lines 227 to 239 in d4bd04d

	for id in stack[index_of_this..].iter() {
	if let Some(id) = id {
	let span = ctx.span(id).expect("Span not found, this is a bug");
	let extensions = span.extensions();
	if let Some(ext) = extensions.get::<ATraceExtension>() {
	self.trace.begin_section(&ext.name);
	} else {
	eprintln!("Unexpectedly had item in stack without ATraceExtension");
	}
	} else {
	self.trace.begin_section(EXTRA_STR);
	}
	}

We should find a way to test this code. This would involve:

1. Enabling mocking AndroidTrace for testing, somehow (either with a trait, or by specifying the function calls directly)
2. Creating a layer which uses this mocked AndroidTrace
3. Running tests with different timings of enabled/disabled and span creation/exiting
4. Saving the resulting trees, and seeing if they make sense (probably with a kind of snapshot testing), + that each open of a span is met with a matching close

This is not a high priority