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