Unbreak VPN unit tests.

VpnTest was broken earlier due to a change to always-on VPN
notifications. This CL adds the corresponding mocks to the unit test to
fix it.

Bug: 36650087
Bug: 65439160
Test: runtest frameworks-net
Change-Id: Icff57c7e927c135d75a7d70ff347a579c5d45134
Merged-In: Icff57c7e927c135d75a7d70ff347a579c5d45134
(cherry picked from commit 3da6a1fc)