Set setCurrentProxyScriptUrl as public

PacManager is part of services.jar, while ProxyManager is in
service-connectivity.jar. setCurrentProxyScriptUrl cannot be
package-private and called across jars.

As part of modularization of service-connectivity.jar, the
PacManager interface will then be formally defined.

Bug: 171540887
Test: Splitting service-connectivity, ProxyManager can reference the
      method
Change-Id: Ie41a5b569a119f305bf647e55a65febbde459352