Move the Thread proguard flags to ConnectivityService's proguard.flags

We needed this proguard flag because
ThreadNetworkService#onLocalNetworkInfoChanged will otherwise be
stripped. We need to keep the method to ensure the border routing
feature works.

Initially we put the flag in thread/service/proguard.flags. However,
this seems not working on udc-mainline-prod branch. After moving it to
Connectivity/service/proguard.flags, it works as desired.

Bug: 313539492

Change-Id: I33cb0602095a617dbb1cfe3d9002f9c68e93cccf