In the tests there may not be a reponse to ftd ping command, and the
function does not check for ping success. So using a short timeout of
0.1s to let the function finish faster.
This change also removed the additional start BR step in
unicastRouting_borderRouterSendsUdpToThreadDevice_datagramReceived as
this is already done in setUp().
Bug: 328144082
Test: atest ThreadNetworkIntegrationTests:android.net.thread.BorderRoutingTest
Change-Id: I18202c74596d4a08bc8238723049e05994cca659
4bf1dcd7f4