[HostStubGen] Use buffered output

Just realized FileOutputStream wasn't buffered. Doh.

This change makes the non-sharded version reasonably faster, but
12x sharding still seems to give a better result, so we'll keep it.

Flag: EXEMPT host side test change only
Bug: 292141694
Test: atest RavenwoodBivalentTest

Change-Id: Ief4f97b1c6f40fb901308aab0a812d731324b002