vndk-def: Compute sp-lib in compute_vndk()

This commit extracts some sp-lib related code from VNDKCommand.main()
into compute_vndk() to simplify the API.

For the unit test, we will run ELFLinker._compute_vndk() so that we can
mock SPLibResult.

Test: ./tests/run.py
Change-Id: Ic43df22a4d178b96a4228097af625b153f8cc5a3
3 files changed