[android] Introduce RunLoop based on Looper

Also implement a Timer and AsyncTask based on Android's Looper.

platform/android/src/async_task.cpp

@@ -0,0 +1,60 @@
+#include "run_loop_impl.hpp"
+
+#include <mbgl/util/async_task.hpp>
+#include <mbgl/util/run_loop.hpp>
+
+#include <atomic>
+#include <functional>
+
+namespace mbgl {
+namespace util {
+
+class AsyncTask::Impl : public RunLoop::Impl::Runnable {
+public:
+    Impl(std::function<void()>&& fn)
+        : task(std::move(fn)) {
+        loop->initRunnable(this);
+    }
+
+    ~Impl() {
+        loop->removeRunnable(this);
+    }
+
+    void maySend() {
+        if (!queued.test_and_set()) {
+            loop->addRunnable(this);
+        }
+    }
+
+    TimePoint dueTime() const override {
+        return due;
+    }
+
+    void runTask() override {
+        loop->removeRunnable(this);
+        queued.clear();
+        task();
+    }
+
+private:
+    // Always expired, run immediately.
+    const TimePoint due = Clock::now();
+
+    RunLoop::Impl* loop = reinterpret_cast<RunLoop::Impl*>(RunLoop::getLoopHandle());
+
+    std::atomic_flag queued = ATOMIC_FLAG_INIT;
+    std::function<void()> task;
+};
+
+AsyncTask::AsyncTask(std::function<void()>&& fn)
+    : impl(std::make_unique<Impl>(std::move(fn))) {
+}
+
+AsyncTask::~AsyncTask() = default;
+
+void AsyncTask::send() {
+    impl->maySend();
+}
+
+} // namespace util
+} // namespace mbgl