Android: Move host thread lock to a separate file

Source/Android/jni/Host.h

@@ -0,0 +1,24 @@
+// Copyright 2023 Dolphin Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+#include <mutex>
+
+#include "Core/Core.h"
+
+// The Core only supports using a single Host thread.
+// If multiple threads want to call host functions then they need to queue
+// sequentially for access.
+struct HostThreadLock
+{
+  static std::mutex s_host_identity_mutex;
+  std::unique_lock<std::mutex> m_lock;
+
+  explicit HostThreadLock() : m_lock(s_host_identity_mutex) { Core::DeclareAsHostThread(); }
+  HostThreadLock(const HostThreadLock& other) = delete;
+  HostThreadLock(HostThreadLock&& other) = delete;
+  HostThreadLock& operator=(const HostThreadLock& other) = delete;
+  HostThreadLock& operator=(HostThreadLock&& other) = delete;
+  ~HostThreadLock() { Core::UndeclareAsHostThread(); }
+};