Derive current servlet/portlet from the current service (#11779)

Implement VaadinPortlet.getCurrent() to use VaadinService.getCurrent()
instead of having a separate thread local variable. This is done to
avoid classloading issues when determining which instances to preserve
in CurrentInstance.setThreadLocals. The two current instances have
previously been kept in sync in all cases except during
VaadinPortlet.init where VaadinService has not yet been created.

VaadinPortlet.setCurrent() is removed as no way of preserving its
semantics has been found. This breaks API compatibility, but is probably
better than having a deprecated implementation that can not work as
expected in all situations.

The same changes have also been made to VaadinServlet to maintain the
symmetry.

Change-Id: I0a1ccc07a4aeecec558a9aaae211bd56207313d8