Fix ChannelsTest

Use public methods in ChannelInputStream to read from Channels
so that we throw the right sort of exception for non-blocking
Selectable channels.

Also amend a few tests that were making bogus assumptions about
isReady(). Channels derived from File input / output streams are
always ready.

Change-Id: I689eec640f6a51e0f92dea906f19d4f8a50959bb