Client API: Allow data sources to handle Stop asynchronously

Exposes the ability to defer tracing session stop to the public
producer-side API. This feature was already part of the tracing
protocol but was not exposed to the public API surface.

Also makes some minor cleanups:
- Rename instance_state->started to trace_lambda_enabled
  to better reflect what the field does.
- Rename {android_,}client_api_example as the code is not really
  Android-specific.

Bug: 132678367
Bug: 137210068
Change-Id: I446d2ce9df1c9d4517a8b078813b9cc4138aef20
diff --git a/BUILD.gn b/BUILD.gn
index f50d0c9..bf85e54 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -63,6 +63,7 @@
       "protos/perfetto/trace:merged_trace",  # For syntax-checking the proto.
       "src/ipc/protoc_plugin:ipc_plugin($host_toolchain)",
       "src/tracing:client_api",
+      "test:client_api_example",
       "tools:protoc_helper",
     ]
     if (perfetto_build_standalone) {