ivi-layout: Use weston_view_move_to_layer()

Use the weston_view_move_to_layer() helper instead of calling
weston_layer_entry_insert() / weston_view_unmap() directly.

This requires us to stop just clearing out the layer view list and then
adding visible surfaces' views to it. Instead, we have to explicitly
move all views onto or off of the layers.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>