netd tun tests - find(":") -> find(':')

Resolves:
  system/netd/tests/tun_interface.cpp:148:19: warning: 'find' called with a string literal consisting of a single character; consider using the more effective overload accepting a character [performance-faster-string-find]

Test: builds
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I1e86cc386859b8a7c20c651d5efd25501dc45e53