Add Swappy & Pre-Transformed Swapchain

- Adds Swappy for Android for stable frame pacing
- Implements pre-transformed Swapchain so that Godot's compositor is in
charge of rotating the screen instead of Android's compositor
(performance optimization for phones that don't have HW rotator)

============================

The work was performed by collaboration of TheForge and Google. I am
merely splitting it up into smaller PRs and cleaning it up.

Changes from original PR:

- Removed "display/window/frame_pacing/android/target_frame_rate" option
to use Engine::get_max_fps instead.
- Target framerate can be changed at runtime using Engine::set_max_fps.
- Swappy is enabled by default.
- Added documentation.
- enable_auto_swap setting is replaced with swappy_mode.

platform/android/display_server_android.h

@@ -124,6 +124,7 @@ public:
 
 	virtual void screen_set_orientation(ScreenOrientation p_orientation, int p_screen = SCREEN_OF_MAIN_WINDOW) override;
 	virtual ScreenOrientation screen_get_orientation(int p_screen = SCREEN_OF_MAIN_WINDOW) const override;
+	virtual int screen_get_internal_current_rotation(int p_screen) const override;
 
 	virtual int get_screen_count() const override;
 	virtual int get_primary_screen() const override;