Refactor setCurrentProxyScriptUrl to a void method

setCurrentProxyScriptUrl is used by ProxyTracker which is
included in connectivity service mainline module. To make
this method @SystemApi in follow-up patch, change this
from a boolean method to a void method. Moving the send or
not send broadcast logic to caller because it can know
whether it needs to send a broadcast or not.

Bug: 177035719
Test: FrameworksNetTests
Change-Id: I7c0fd20bb449463a65952cfa9fb8a42310da079c