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
78cc340c2d