Fixed wrong behaviors with HW back button.

Fixed wrong behaviors with HW back button.

1. If IME is shown, context menu and handlers shouldn't be hidden by H/W back
key. Currently context menu and handlers are cleared regardless of IME state
becuase |SelectionControllerEfl::HideHandleAndContextMenu()| is called in
|RWHVEfl::ImeCompositionRangeChanged| which is called when IME is shown or
hidden. This patch removes it.

2. HW back key deletes selected text. |IMContextEfl::OnPreeditChanged| isn't
returned propery when buffer is empty.

Bug: http://107.108.218.239/bugzilla/show_bug.cgi?id=15025
Bug: http://107.108.218.239/bugzilla/show_bug.cgi?id=15060

Reviewed by: a1.gomes, djmix.kim, g.czajkowski

Change-Id: If85f59f0121cf36159e4b5d0ea9adda6a9df78d6
Signed-off-by: Sanghyup Lee <sh53.lee@samsung.com>