[eche] adding foundation for mini launcher for streamable apps.

In this cl, a new view is added to the phone hub that is activated based on the current state of the ui. It will show a grid of apps. However, in this cl, we just show some placeholders instead of app icons.
I had to make some changes to remove the header of the phone hub as well  when we are showing this new state.

Bug filed to add tests: b/259305141
Low-Coverage-Reason:A separate task was added to add tests. It is treated as a blocker for our launch.

TEST=Locally enabled the feature by hardcoding it and making sure it works. Also, disabled the feature and made sure that Phone hub works normally.

Bug: b/258300558
Change-Id: If9e27068ab0536de77b6ce010266171cebf3e662
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4029476
Commit-Queue: Abbas Nayebi <nayebi@google.com>
Reviewed-by: Jon Mann <jonmann@chromium.org>
Auto-Submit: Abbas Nayebi <nayebi@google.com>
Cr-Commit-Position: refs/heads/main@{#1073102}

diff --git a/ash/system/phonehub/app_stream_launcher_view.h b/ash/system/phonehub/app_stream_launcher_view.h
new file mode 100644
index 0000000..c472285a
--- /dev/null
+++ b/ash/system/phonehub/app_stream_launcher_view.h

@@ -0,0 +1,66 @@
+// Copyright 2022 The Chromium Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef ASH_SYSTEM_PHONEHUB_APP_STREAM_LAUNCHER_VIEW_H_
+#define ASH_SYSTEM_PHONEHUB_APP_STREAM_LAUNCHER_VIEW_H_
+
+#include <memory>
+#include "ash/ash_export.h"
+#include "ash/system/phonehub/phone_hub_content_view.h"
+#include "ui/gfx/vector_icon_types.h"
+#include "ui/views/controls/button/button.h"
+#include "ui/views/view.h"
+
+namespace views {
+class Button;
+}
+namespace ash {
+
+namespace phonehub {
+class PhoneHubManager;
+}
+
+// A view of the Phone Hub panel, displaying the apps that user can launch for
+// app streaming.
+class ASH_EXPORT AppStreamLauncherView : public PhoneHubContentView {
+ public:
+  explicit AppStreamLauncherView(phonehub::PhoneHubManager* phone_hub_manager);
+  ~AppStreamLauncherView() override;
+
+  // views::View:
+  void ChildPreferredSizeChanged(View* child) override;
+  void ChildVisibilityChanged(View* child) override;
+  const char* GetClassName() const override;
+
+  // PhoneHubContentView:
+  phone_hub_metrics::Screen GetScreenForMetrics() const override;
+
+ private:
+  friend class AppStreamLauncherViewTest;
+  FRIEND_TEST_ALL_PREFIXES(AppStreamLauncherViewTest, OpenView);
+
+  std::unique_ptr<views::View> CreateAppListView();
+  std::unique_ptr<views::View> CreateHeaderView();
+  std::unique_ptr<views::Button> CreateButton(
+      views::Button::PressedCallback callback,
+      const gfx::VectorIcon& icon,
+      int message_id);
+  std::unique_ptr<views::View> CreateViewForItemAtIndex(size_t index);
+  void AppIconActivated();
+
+  // Handles the click on the "back" arrow in the header.
+  void OnArrowBackActivated();
+
+  views::Button* arrow_back_button_ = nullptr;
+  phonehub::PhoneHubManager* phone_hub_manager_;
+
+  // Contains all the apps
+  views::View* items_container_;
+
+  base::WeakPtrFactory<AppStreamLauncherView> weak_factory_{this};
+};
+
+}  // namespace ash
+
+#endif  // ASH_SYSTEM_PHONEHUB_APP_STREAM_LAUNCHER_VIEW_H_