Android: Added basic drop file support (thanks, "noxalus"!).

This lets SDL-based apps respond to "Open With" commands properly, as they
can now obtain the requested path via a standard SDL dropfile event.

This is only checked on startup, so apps don't get drop events at any other
time, even if Android supports that, but this is still a definite
improvement.

Fixes Bugzilla #2762.

src/core/android/SDL_android.c

@@ -141,6 +141,16 @@ JNIEXPORT void JNICALL SDL_Android_Init(JNIEnv* mEnv, jclass cls)
     __android_log_print(ANDROID_LOG_INFO, "SDL", "SDL_Android_Init() finished!");
 }
 
+/* Drop file */
+void Java_org_libsdl_app_SDLActivity_onNativeDropFile(
+                                    JNIEnv* env, jclass jcls,
+                                    jstring filename)
+{
+    const char *path = (*env)->GetStringUTFChars(env, filename, NULL);
+    SDL_SendDropFile(path);
+    (*env)->ReleaseStringUTFChars(env, filename, path);
+}
+
 /* Resize */
 JNIEXPORT void JNICALL Java_org_libsdl_app_SDLActivity_onNativeResize(
                                     JNIEnv* env, jclass jcls,