Added detection of touch devices before first touch events happen on Android.

On Android available touch devices are now added with video initialization (like
the keyboard). This fixes SDL_GetNumTouchDevices() returning 0 before any touch
events happened although there is a touch screen available. The adding of touch
devices after a touch event was received is still active to allow connecting
devices later (if this is possible) and to provide a fallback if the new init
did not work somehow. For the implementation JNI was used and API level 9 is
required. There seems to be nothing in the Android NDK's input header (input.h)
to implement everything on C side without communication with Java side.

src/video/android/SDL_androidtouch.c

@@ -31,6 +31,7 @@
 
 #include "SDL_androidtouch.h"
 
+#include "../../core/android/SDL_android.h"
 
 #define ACTION_DOWN 0
 #define ACTION_UP 1
@@ -53,6 +54,19 @@ static void Android_GetWindowCoordinates(float x, float y,
     *window_y = (int)(y * window_h);
 }
 
+void Android_InitTouch(void)
+{
+    int i;
+    int* ids;
+    int number = Android_JNI_GetTouchDeviceIds(&ids);
+    if (0 < number) {
+        for (i = 0; i < number; ++i) {
+            SDL_AddTouch((SDL_TouchID) ids[i], ""); /* no error handling */
+        }
+        SDL_free(ids);
+    }
+}
+
 void Android_OnTouch(int touch_device_id_in, int pointer_finger_id_in, int action, float x, float y, float p)
 {
     SDL_TouchID touchDeviceId = 0;

src/video/android/SDL_androidtouch.h

@@ -22,6 +22,7 @@
 
 #include "SDL_androidvideo.h"
 
+extern void Android_InitTouch(void);
 extern void Android_OnTouch( int touch_device_id_in, int pointer_finger_id_in, int action, float x, float y, float p);
 
 /* vi: set ts=4 sw=4 expandtab: */

src/video/android/SDL_androidvideo.c

@@ -36,6 +36,7 @@
 #include "SDL_androidclipboard.h"
 #include "SDL_androidevents.h"
 #include "SDL_androidkeyboard.h"
+#include "SDL_androidtouch.h"
 #include "SDL_androidwindow.h"
 
 #define ANDROID_VID_DRIVER_NAME "Android"
@@ -165,6 +166,8 @@ Android_VideoInit(_THIS)
 
     Android_InitKeyboard();
 
+    Android_InitTouch();
+
     /* We're done! */
     return 0;
 }