Fix the crash that happens when touch device is re-enabled.

This patch allows to destroy the wl_touch interface by calling
wl_touch_destroy when touch device is un-plugged or disabled,
which prevents Wayland from passing a dangling pointer of
WaylandTouchscreen to touch event handlers.

Bug: TC-1729
Fix the crash that happens when touch device is re-enabled.

This is part of previous changes. We need to call wl_pointer_destroy to release the wl_pointer device when pointer(mouse) device is disabled.

BUT=TC-1729
Merge pull request #307 from yejingfu/bug-fix

Fix crash on touch event