Improve the setUnderlyingNetwork method by throwing IAE
when Network's LinkProperties is null. Also update the
API doc to give caller more guidance in using this API
Bug: 232309601
Test: UT: IpSecServiceParameterizedTest, IpSecServiceTest,
IpSecServiceRefcountedResourceTest, IpSecManagerTest, VpnTest
Test: CTS: IpSecManagerTest, IpSecManagerTunnelTest, Ikev2VpnTest
Test: make doc-comment-check-docs
Change-Id: Idab4706b0db42ed2222fb48b168589ed005d2f2f
be3eb3d56a