Implement "take a screenshot" feature

* Like in android - press power+volume down, screenshots are saved
  in /sdcard/Pictures/Screenshots (if /sdcard is mounted) or /tmp

Change-Id: Iaefa15b11a1d5fdfac57d77388db1621f378a8d4
Signed-off-by: Vojtech Bocek <vbocek@gmail.com>
diff --git a/minuitwrp/Android.mk b/minuitwrp/Android.mk
index 2797a00..3c148ea 100644
--- a/minuitwrp/Android.mk
+++ b/minuitwrp/Android.mk
@@ -2,7 +2,7 @@
 
 include $(CLEAR_VARS)
 
-LOCAL_SRC_FILES := events.c resources.c graphics_overlay.c
+LOCAL_SRC_FILES := events.c resources.c graphics_overlay.c graphics_utils.c
 
 ifneq ($(TW_BOARD_CUSTOM_GRAPHICS),)
     LOCAL_SRC_FILES += $(TW_BOARD_CUSTOM_GRAPHICS)