genlist: performance enhancement by refactoring realization logics

genlist: performance enhancement by refactoring realization logics

item realize and unrealize time can be reduced by removing
unnecessary view / proxy object creation.

we have cache for unrealized view / proxy,
but it will be filled by unrealization,
so we need to place item realization after unrealization.

this patch is tweak of change all the item realization
performed right after the every item unrealization done.

if there are so many unrealization,
cache can be overflow, which is double of block size.

but most of cases,
visible item counts are less than block size,
and worst scenario of screen refresh is every visible item changes,
so cache size will be big enough to perform the 1 frame unrealizaiton.

Change-Id: Ibad2f48efca829137d3a95b4110a998eb7533692