Add an implementation of SipTransportImplBase for testing

Add SipTransportImplBase implementation for testing in the
ImsTestApp.

Test: manual with ImsTestApp
Merged-In: I740d1f3c7ea61072fb4b119144cdfbf7f3f181b0
Change-Id: I740d1f3c7ea61072fb4b119144cdfbf7f3f181b0
3 files changed