interface_scrollable: Compensate scroller when scroller is not stable

interface_scrollable: Compensate scroller when scroller is not stable

Scroller internal states could be invalid when scroller is being calculated.
However, user app code could call scroller apis in middle of calculating of the scroller,
which mean apis are excuted on the invalid states of the scroller.
In that case, scroller api couldn't make what they want to do at all.

This patch compensates the current "Scroller Logic" when getting a more
exact page infomation from scroller while the "Scroller" is calculated.

@tizen_fix

Change-Id: If14ec3dac225dcc95f0666af7e0639a2958c667c
Signed-off-by: Wonki Kim <wonki_.kim@samsung.com>