InputTracer: Add tests for perfetto backend

Add a new InputTraceSession class that encapsulates the logic to take
and decode a perfetto input trace for testing.

Then, use it to add new tests to verify the behavior of entire tracing
framework, using InputDispatcher as the interface.

Bug: 210460522
Test: atest inputflinger_tests
Change-Id: I5a9b47e831c5c30e5d7f2b60c9b8075b7d330e9e
diff --git a/services/inputflinger/dispatcher/trace/InputTracingPerfettoBackend.h b/services/inputflinger/dispatcher/trace/InputTracingPerfettoBackend.h
index e945066..fdfe495 100644
--- a/services/inputflinger/dispatcher/trace/InputTracingPerfettoBackend.h
+++ b/services/inputflinger/dispatcher/trace/InputTracingPerfettoBackend.h
@@ -51,6 +51,8 @@
 public:
     using GetPackageUid = std::function<gui::Uid(std::string)>;
 
+    static bool sUseInProcessBackendForTest;
+
     explicit PerfettoBackend(GetPackageUid);
     ~PerfettoBackend() override = default;
 
@@ -61,6 +63,7 @@
 private:
     // Implementation of the perfetto data source.
     // Each instance of the InputEventDataSource represents a different tracing session.
+    // Its lifecycle is controlled by perfetto.
     class InputEventDataSource : public perfetto::DataSource<InputEventDataSource> {
     public:
         explicit InputEventDataSource();