Unify scrollable list code

The goal of this patch set is to eliminate the code duplication
caused by copy/paste of the code in the file selector, listbox,
and partition list GUI elements. Those classes will now utilize a
single GUIScrollList class that will handle rendering and
scrolling.

Change-Id: I0cb98ab36cf47178296034293435225658c779cd

gui/Android.mk

@@ -25,7 +25,8 @@ LOCAL_SRC_FILES := \
     input.cpp \
     blanktimer.cpp \
     partitionlist.cpp \
-    mousecursor.cpp
+    mousecursor.cpp \
+    scrolllist.cpp
 
 ifneq ($(TWRP_CUSTOM_KEYBOARD),)
   LOCAL_SRC_FILES += $(TWRP_CUSTOM_KEYBOARD)