Make RttTextStream's read interruptible

Wrap the FileInputStream in a java.nio.Channel so that sending a
Thread.interrupt() to the thread blocked on the read will actually do
something.

Change-Id: Icc11ba69167f448e2b33d9a1a13a1dfa5e5d0d58
Fixes: 110570772
Test: manual (follow repro steps in bug, but type really fast),treehugger