Add Android backend and example (#3446)

examples/example_android_opengl3/android/app/src/main/AndroidManifest.xml

@@ -0,0 +1,24 @@
+<?xml version="1.0" encoding="utf-8"?>
+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+    package="imgui.example.android">
+
+    <application
+        android:label="ImguiExample"
+        android:allowBackup="false"
+        android:fullBackupContent="false"
+        android:hasCode="true">
+
+        <activity
+            android:name="imgui.example.android.MainActivity"
+            android:theme="@android:style/Theme.NoTitleBar.Fullscreen"
+            android:configChanges="orientation|keyboardHidden|screenSize">
+            <meta-data android:name="android.app.lib_name"
+                android:value="ImguiExample" />
+
+            <intent-filter>
+                <action android:name="android.intent.action.MAIN" />
+                <category android:name="android.intent.category.LAUNCHER" />
+            </intent-filter>
+        </activity>
+    </application>
+</manifest>

examples/example_android_opengl3/android/app/src/main/java/MainActivity.kt

@@ -0,0 +1,40 @@
+package imgui.example.android
+
+import android.app.NativeActivity
+import android.os.Bundle
+import android.content.Context
+import android.view.inputmethod.InputMethodManager
+import android.view.KeyEvent
+import java.util.concurrent.LinkedBlockingQueue
+
+class MainActivity : NativeActivity() {
+    public override fun onCreate(savedInstanceState: Bundle?) {
+        super.onCreate(savedInstanceState)
+    }
+
+    fun showSoftInput() {
+        val inputMethodManager = getSystemService(Context.INPUT_METHOD_SERVICE) as InputMethodManager
+        inputMethodManager.showSoftInput(this.window.decorView, 0)
+    }
+
+    fun hideSoftInput() {
+        val inputMethodManager = getSystemService(Context.INPUT_METHOD_SERVICE) as InputMethodManager
+        inputMethodManager.hideSoftInputFromWindow(this.window.decorView.windowToken, 0)
+    }
+
+    // Queue for the Unicode characters to be polled from native code (via pollUnicodeChar())
+    private var unicodeCharacterQueue: LinkedBlockingQueue<Int> = LinkedBlockingQueue()
+
+    // We assume dispatchKeyEvent() of the NativeActivity is actually called for every
+    // KeyEvent and not consumed by any View before it reaches here
+    override fun dispatchKeyEvent(event: KeyEvent): Boolean {
+        if (event.action == KeyEvent.ACTION_DOWN) {
+            unicodeCharacterQueue.offer(event.getUnicodeChar(event.metaState))
+        }
+        return super.dispatchKeyEvent(event)
+    }
+
+    fun pollUnicodeChar(): Int {
+        return unicodeCharacterQueue.poll() ?: 0
+    }
+}