libusbhost: factor out frameworks descriptors -> byte[] code.

Sony needed to increase the limit for a UVC device with > 4KiB
descriptors, but there wasn't one clean place to do that. Tidy up, and
also go straight to *16KiB* which was the largest buffer any copy of
this code was using to date.

Test: treehugger
Change-Id: Ide41a217dcf4291a7d38836264f59e06967d9d91