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/Android.bp.extras b/Android.bp.extras
index a2db5ed..3d89bce 100644
--- a/Android.bp.extras
+++ b/Android.bp.extras
@@ -72,7 +72,7 @@
 cc_binary {
   name: "libperfetto_client_example",
   srcs: [
-    "test/android_client_api_example.cc",
+    "test/client_api_example.cc",
   ],
   static_libs: [
     "libperfetto_client_experimental",