Fix flake : testVpnSetUnderlyingNetworks

When the device is very slow to run this test, cell might end
its lingering period in the middle of the test while wifi is
the default. VPN will then ignore it as an underlying network,
causing the test to fail.

Test: testVpnSetUnderlyingNetworks
      Added sleep in a strategic place to make sure the problem
      was correctly identified, and observed the test failing.
      After the fix, the test passes with the sleep.
Fixes: 280627836
Fixes: 293844384
Change-Id: Iaf56de963eb17845c1d76a0310b55f7f9321f7e4