app/internal/headless: [Android] expose Java initialization method

Android apps may want to run Go code independent of the Gio Android Activity.
Expose a Gio.init Java method public for early loading and initialization of
the Go library.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/internal/window/Gio.java

@@ -0,0 +1,39 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package org.gioui;
+
+import android.content.Context;
+
+import java.io.UnsupportedEncodingException;
+
+public class Gio {
+	private final static Object initLock = new Object();
+	private static boolean jniLoaded;
+
+	/**
+	 * init loads and initializes the Go native library and runs
+	 * the Go main function.
+	 *
+	 * It is exported for use by Android apps that need to run Go code
+	 * outside the lifecycle of the Gio activity.
+	 */
+	public static synchronized void init(Context appCtx) {
+		synchronized (initLock) {
+			if (jniLoaded) {
+				return;
+			}
+			String dataDir = appCtx.getFilesDir().getAbsolutePath();
+			byte[] dataDirUTF8;
+			try {
+				dataDirUTF8 = dataDir.getBytes("UTF-8");
+			} catch (UnsupportedEncodingException e) {
+				throw new RuntimeException(e);
+			}
+			System.loadLibrary("gio");
+			runGoMain(dataDirUTF8, appCtx);
+			jniLoaded = true;
+		}
+	}
+
+	static private native void runGoMain(byte[] dataDir, Context context);
+}