[ipsec-qtaguid] Tag sockets upon creation of encap sockets

Added calls to tag encap sockets to that of the UID for which the encap
socket is being created on behalf of. This ensures that all data
accounting generated for the UDP-encap-ESP socket is correctly billed to
the right UID.

Bug: 62994731
Test: New tests added to IpSecServiceTest.java, passing
Change-Id: I15365ea9c982fd7b4e3cdeff314ddfba2289c86e