perfetto: split up FtraceProducer into its own file to allow reuse

We will reuse FtraceProducer in integration tests and we don't want to
call main to do so.

Split it off and add a source set to allow this to work correctly.

Bug: None
Change-Id: I4461575c1bedec72e5bc08a6e059ccecbec945c1
diff --git a/Android.bp b/Android.bp
index 1ecc372..d7ee548 100644
--- a/Android.bp
+++ b/Android.bp
@@ -47,6 +47,7 @@
     "src/protozero/protozero_message_handle.cc",
     "src/protozero/scattered_stream_writer.cc",
     "src/traced/perfetto_cmd/perfetto_cmd.cc",
+    "src/traced/probes/ftrace_producer.cc",
     "src/traced/probes/probes.cc",
     "src/traced/service/service.cc",
     "src/tracing/core/chunked_protobuf_input_stream.cc",