Handle read-only portals in ArrayHandleMultiplexer

The ArrayPortalMultiplexer always called Set on the delegate portals.
However, sometimes the portals were not supported. Instead, only call
when the delegate is supported.

Probably the "right" thing to do would be to check all posible portals
for support, but that sounds like it would slow down the compile.
7 jobs for multiplexer-fixes-a