Add an implementation of SipTransportImplBase for testing am: d2da7b2861 am: 8ad242f9c5 am: 1c73293cd6

Original change: https://android-review.googlesource.com/c/platform/packages/services/Telephony/+/1453559

Change-Id: I5c62ccda7dea1e00976584483f2a6ac849abe621