build: reorg jni files

app/src/main/cpp/jni_mpg123.cpp

@@ -0,0 +1,19 @@
+//
+// Created by david on 02.03.2026.
+//
+
+#include <jni.h>
+#include "mpg123.h"
+#include <oboe/Oboe.h>
+
+extern "C" {
+
+// nice-to: merge with lockstep.cpp
+
+JNIEXPORT jint JNICALL
+Java_at_lockstep_pb_PlaybackEngine_native_1mpg123_1init
+        (JNIEnv *env, jclass) {
+    return mpg123_init();
+}
+
+}