Add Android backend and example (#3446)

docs/BACKENDS.md

@@ -57,6 +57,7 @@ In the [backends/](https://github.com/ocornut/imgui/blob/master/backends) folder
 
 List of Platforms Backends:
 
+    imgui_impl_android.cpp    ; Android native app API
     imgui_impl_glfw.cpp       ; GLFW (Windows, macOS, Linux, etc.) http://www.glfw.org/
     imgui_impl_osx.mm         ; macOS native API (not as feature complete as glfw/sdl backends)
     imgui_impl_sdl.cpp        ; SDL2 (Windows, macOS, Linux, iOS, Android) https://www.libsdl.org

docs/EXAMPLES.md

@@ -79,6 +79,10 @@ Changelog, so if you want to update them later it will be easier to catch up wit
 Allegro 5 example. <BR>
 = main.cpp + imgui_impl_allegro5.cpp
 
+[example_android_opengl3/](https://github.com/ocornut/imgui/blob/master/examples/example_android_opengl3/) <BR>
+Android + OpenGL3 (ES) example. <BR>
+= main.cpp + imgui_impl_android.cpp + imgui_impl_opengl3.cpp
+
 [example_apple_metal/](https://github.com/ocornut/imgui/blob/master/examples/example_metal/) <BR>
 OSX & iOS + Metal example. <BR>
 = main.m + imgui_impl_osx.mm + imgui_impl_metal.mm <BR>

docs/README.md

@@ -117,7 +117,7 @@ Integrating Dear ImGui within your custom engine is a matter of 1) wiring mouse/
 
 Officially maintained backends/bindings (in repository):
 - Renderers: DirectX9, DirectX10, DirectX11, DirectX12, Metal, OpenGL/ES/ES2, Vulkan, WebGPU.
-- Platforms: GLFW, SDL2, Win32, Glut, OSX.
+- Platforms: GLFW, SDL2, Win32, Glut, OSX, Android.
 - Frameworks: Emscripten, Allegro5, Marmalade.
 
 [Third-party backends/bindings](https://github.com/ocornut/imgui/wiki/Bindings) wiki page: