Offer to stream and fsync() install sessions.

Installers are interested in both streaming APK data and establishing
a happens-after relationship to support resuming downloads after a
process kill or battery pull.

This exposes a generic OutputStream for writing, and hooks up flush()
to be a blocking call which returns only when all outstanding write()
data has been fsync()'ed to disk.

Tests to verify behavior.

Bug: 14975160
Change-Id: I38289867c80ac659163bb0c2158ef12d99cc570d