Ladybird/Android: Add EventLoopImplementation for ALooper

Timers run in their own thread, to take advantage of existing Java
Executor features. By hooking into ALooper, we can spin the main
Activity's UI thread event loop without causing a fuss, or spinning the
CPU by just polling our event loop constantly.

Ladybird/Android/src/main/cpp/TimerExecutorService.cpp

@@ -0,0 +1,29 @@
+/*
+ * Copyright (c) 2023, Andrew Kaster <akaster@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "ALooperEventLoopImplementation.h"
+#include <LibCore/EventLoop.h>
+#include <jni.h>
+
+extern "C" JNIEXPORT void JNICALL Java_org_serenityos_ladybird_TimerExecutorService_00024Timer_nativeRun(JNIEnv*, jobject /* thiz */, jlong native_data, jlong id)
+{
+    auto& thread_data = *reinterpret_cast<Ladybird::EventLoopThreadData*>(native_data);
+
+    if (auto timer_data = thread_data.timers.get(id); timer_data.has_value()) {
+        auto receiver = timer_data->receiver.strong_ref();
+        if (!receiver)
+            return;
+
+        if (timer_data->visibility == Core::TimerShouldFireWhenNotVisible::No)
+            if (!receiver->is_visible_for_timer_purposes())
+                return;
+
+        Core::TimerEvent event(id);
+
+        // FIXME: Should the dispatch happen on the thread that registered the timer?
+        receiver->dispatch_event(event);
+    }
+}