Upgrade FDT to use promises instead of old syntax

ok: and err: functions are no longer supported, I think, so of
course they never got called. Meaning large chunked uploads never
finished.

Bug: 60821
Change-Id: Iaf20c08a025039402e498baa24c01fb9e55c4e6b
(cherry picked from commit 2981edb5735ac6312e5ccbda502c312bf414fd3d)