When there are thread devices with DUA in the thread network, BR itself
may not have a DUA address, so it doesn't have route to DUA in the
thread-wpan routing table. When thread device sends a packet from the
DUA to AIL via BR, BR rpfilter would drop the packet. Adding the route
to DUA prefix in thread-wpan solves this problem.
Bug: 335762753
Test: atest ThreadNetworkIntegrationTests:android.net.thread.ThreadIntegrationTest#addPrefixToNetData_routeIsAddedToTunInterface
atest ThreadNetworkIntegrationTests:android.net.thread.ThreadIntegrationTest#removePrefixFromNetData_routeIsRemovedFromTunInterface
Change-Id: I5ca8f7e723116a63718734f72f1b463d9c380329
14de932e9b