SdkSpec.Version was an int type. Now it becomes ApiLevel type which
is a better abstraction of the version (or api level).
Bug: 1655587
Test: m
Change-Id: I4d67b9b9eae45f653b6af4f5b73da9e091b3dfab
... in preparation for making the handling of sdk versions consistent
across java and cc modules.
Bug: 175678607
Test: m
Change-Id: I598f0454bce9b7320621022115412fbe97403945