Flush output buffers when command is complete

The PrintWriters in ShellCommand are not autoflush, so they need to be manually
flushed. Rather than put the onus on subclasses, this automatically flushes them
when the command exits cleanly

Test: Run a command that has an exception. See the exception get printed to the
console after this change.

Change-Id: I6f27a4d1183008191ee97071c2e238af7a2edd5f