Add experimental Android support (#190)

* Add `Raylib-cs.Android` project
* Update ci to handle android native libraries
* Update ci to pack android project

Raylib-cs.Android/Raylib-cs.Android.csproj

@@ -0,0 +1,15 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFrameworks>net6.0-android;net7.0-android</TargetFrameworks>
+    <SupportedOSPlatformVersion>21</SupportedOSPlatformVersion>
+    <RootNamespace>Raylib_cs</RootNamespace>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="../Raylib-cs/Raylib-cs.csproj" />
+  </ItemGroup>
+
+</Project>

Raylib-cs.Android/RaylibActivity.cs

@@ -0,0 +1,17 @@
+using System.Runtime.InteropServices;
+
+namespace Raylib_cs;
+
+public abstract class RaylibActivity : NativeActivity
+{
+    protected override void OnCreate(Bundle? savedInstanceState)
+    {
+        base.OnCreate(savedInstanceState);
+        RaylibSetAndroidCallback(OnReady);
+    }
+
+    protected abstract void OnReady();
+
+    [DllImport(Raylib.NativeLibName, CallingConvention = CallingConvention.Cdecl)]
+    private static extern void RaylibSetAndroidCallback(Action callback);
+}

Raylib-cs.Android/android.patch

@@ -0,0 +1,27 @@
+diff --git a/src/rcore.c b/src/rcore.c
+index eae4951..4400b22 100644
+--- a/src/rcore.c
++++ b/src/rcore.c
+@@ -1,3 +1,22 @@
++#if defined (PLATFORM_ANDROID)
++#include <stdlib.h>
++
++typedef void (*RaylibAndroidCallback)();
++static RaylibAndroidCallback _androidCallback = NULL;
++
++void RaylibSetAndroidCallback(RaylibAndroidCallback callback) {
++  _androidCallback = callback;
++}
++
++int main(int argc, char *argv[]) {
++  (void)argc;
++  (void)argv;
++  while (_androidCallback == NULL) {
++  }
++  _androidCallback();
++}
++#endif
++
+ /**********************************************************************************************
+ *
+ *   rcore - Basic functions to manage windows, OpenGL context and input on multiple platforms