Use android::base::Pipe.

Also clean up the comments regarding updater/recovery protocol.

Test: Sideload a package on taimen. Check that /tmp/recovery.log
      contains the log from update_engine_sideload.
Test: Factory reset on taimen. Check that log file has the info from
      mke2fs.
Change-Id: If560269ce7007769fc85a63bd228813bdd75bebd