decoder/Client: use std::chrono::duration<double> instead of raw double

test/DumpDecoderClient.hxx

@@ -50,7 +50,7 @@ public:
 	void SeekError() override;
 	InputStreamPtr OpenUri(const char *uri) override;
 	size_t Read(InputStream &is, void *buffer, size_t length) override;
-	void SubmitTimestamp(double t) override;
+	void SubmitTimestamp(FloatDuration t) override;
 	DecoderCommand SubmitData(InputStream *is,
 				  const void *data, size_t length,
 				  uint16_t kbit_rate) override;