Cleanup of the android clipboard handling

Currently there's an unguarded class member for the clipboard, allocated
but unused and not deleted in the destructor. The getter creates a
static clipboard allocated on the heap. This patch aims to add the
missing guards as well as use the class member.

Task-number: QTBUG-36025


Change-Id: I86969390eebcd67b65707e3ecbd4b3be15c7dadb
Reviewed-by: BogDan Vatra <bogdan@kde.org>