Support the full length of USB serial numbers.

Two bugs: we couldn't report the serial number correctly if it was long
enough, and it wasn't possible to connect to a device whose serial number
was long enough to overflow a different fixed-length buffer.

Bug: http://b/20317730

(cherry picked from commit 9309ecbc)

Change-Id: I04c160a215418a57bd4fb27b7f63060c8be6f12e