Update NetworkStack version check in NetworkMonitor.

NetworkMonitor calls INetworkMonitorCallbacks#notifyNetworkTested when
network validation is complete. NetworkMonitor will transition to using
to NetworkStack Stable AIDL version 6. However, the current version
check changes behavior for version 5, which is incorrect.

Bug: 152931568
Test: atest NetworkStackTests
Change-Id: Ib88b8b0c0b23079287e72e50cbe840fa8c642047