Add wifi util lib to classpath for wifi stubs

Fixes the individual wifi stubs to match those in the monolithic
current.txt.

Bug: 152857194
Test: diff <(cat api/system-current.txt | grep 'class RttManager {' -A30) \
           <(cat wifi/api/system-current.txt | grep 'class RttManager {' -A30) \
      && echo pass || echo fail
Change-Id: I3fc8a87e83f8dfee8d97966ead379d7acf0807bc