bind/java: initialize seq machinery from Java

This ensures that the java bindings are ready before any calls are
made by user code. As a bonus, the JNIEnv* is from the Seq class so I
believe no tricks are required to find the right class loader.

Fixes golang/go#10903.

Change-Id: I33b3b39cef6cc2da36e271de882ba8d26610ea34
Reviewed-on: https://go-review.googlesource.com/10296
Reviewed-by: Elias Naur <elias.naur@gmail.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

bind/java/Seq.java

@@ -66,6 +66,12 @@ public class Seq {
 		return tracker.get(refnum);
 	}
 
+	static {
+		initSeq();
+	}
+
+	static native void initSeq();
+
 	// Informs the Go ref tracker that Java is done with this ref.
 	static native void destroyRef(int refnum);