Add small user-event abstraction interface.

Preparation for future UI frontend/backend separation.

This is done to ensure that the code under android/skin/ does not depend
on any QEMU-specific header. We achieve this by adding a new abstract header
"user-events.h" and one QEMU-specific implementations for the functions
defined here.

This also modifies console.h and vl-android.c to make them closer to
upstream (by removing Android-specific changes).

+ fix Makefile.android to always build SDL from sources in standalone mode.

Change-Id: I0d152741e7bb2c9cd283f5c35bd054385c7c1eb3
diff --git a/android/console.c b/android/console.c
index 8eb1497..c6b8a4e 100644
--- a/android/console.c
+++ b/android/console.c
@@ -47,6 +47,7 @@
 #include <fcntl.h>
 #include "android/hw-events.h"
 #include "android/skin/keyboard.h"
+#include "user-events.h"
 
 #if defined(CONFIG_SLIRP)
 #include "libslirp.h"
@@ -1701,7 +1702,7 @@
             return -1;
         }
 
-        kbd_generic_event( type, code, value );
+        user_event_generic( type, code, value );
         p = q;
     }
     return 0;