Fix release build

When we reference an assembly that doesn't get build by the FW
build process we should set the HintPath to the location where
the assembly is stored, not to Outuput/Debug. For Chorus (as well
as Palaso) this is the Downloads directory.

The referenced assembly will be copied to Output/Debug during the
build, but only if it can be found in the source (hintpath)
directory. Relying on it being already in Output/Debug is a bad
idea because the order in which the targets get build may change.

Change-Id: I3dc66290602fd6230240866bd61e9b2a187f71b2