Revert "Fixing memory leak in RenderBufferCache"

This reverts commit faeb5b9932af3c6642bb269ce9be0df2e804efb0.
Avoid to skip app window due to un-assigned sending to bottom state while app window sort

should assign value of WindowToken.sendingToBottom according to the state of this token is moved to top or bottom
to avoid app window is skiped incorrectly due to sending to bottom state is un-assigned while app window sort
and finding focus window.

Change-Id: I190838d1d01ca229155d5176930178ab41335556
Signed-off-by: tingna_sung <tingna_sung@htc.com>