fix selection issue in OnCursorKeyEvent

fix selection issue in OnCursorKeyEvent

Solved the problem when selecting with Shift + Left, Right.

1. Deselect the left == right case.
Previously, the selection handle was drawn even though nothing was selected, which looked weird.

2. For SelectWholeText and SelectText, set PrimaryCursorPosition to RightSelectionPosition.
Selection processing in OnCursorKeyEvent is implemented assuming that the cursor position is RightSelectionPosition.

mPrimaryCursorPosition = mRightSelectionPosition;
This ensures the above behavior.

Change-Id: I1208e37c93e9941319bc1b88502632800129e4d8
Signed-off-by: Bowon Ryu <bowon.ryu@samsung.com>