Un-finalize IpSecManager

Allows VCN and IKE unit tests to mock IpSecManager and thus stop
depending on the hidden APIs in the Connectivity module
(e.g. IpSecService binder interface). Otherwise, any changes on
these related hidden APIs will require topic changes across multiple
projects.

Bug: 217572993
Test: make update-api
Test: atest IpSecManagerTest, IpSecManagerTunnelTest
Change-Id: I29f3ac96f83b4271f29a7051d33ccf47a0dd6c57