Add synchronized to make sure data correctness

Both mNetworkAgent and mConfig are mutable, add synchronized to
make sure the data correctness of those two vars when getNetId()
and getVpnConfig() are called.

Bug: 168576152
Test: atest FrameworkNetTests
      atest CtsHostsideNetworkTests:HostsideVpnTests
Change-Id: I4133de84c904009b5fbf4208853773fc79fdd17f