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.

core/config/project_settings.cpp

@@ -1485,6 +1485,10 @@ ProjectSettings::ProjectSettings() {
 	GLOBAL_DEF("display/window/subwindows/embed_subwindows", true);
 	// Keep the enum values in sync with the `DisplayServer::VSyncMode` enum.
 	custom_prop_info["display/window/vsync/vsync_mode"] = PropertyInfo(Variant::INT, "display/window/vsync/vsync_mode", PROPERTY_HINT_ENUM, "Disabled,Enabled,Adaptive,Mailbox");
+
+	GLOBAL_DEF("display/window/frame_pacing/android/enable_frame_pacing", true);
+	GLOBAL_DEF(PropertyInfo(Variant::INT, "display/window/frame_pacing/android/swappy_mode", PROPERTY_HINT_ENUM, "pipeline_forced_on,auto_fps_pipeline_forced_on,auto_fps_auto_pipeline"), 2);
+
 	custom_prop_info["rendering/driver/threads/thread_model"] = PropertyInfo(Variant::INT, "rendering/driver/threads/thread_model", PROPERTY_HINT_ENUM, "Single-Unsafe,Single-Safe,Multi-Threaded");
 	GLOBAL_DEF("physics/2d/run_on_separate_thread", false);
 	GLOBAL_DEF("physics/3d/run_on_separate_thread", false);