Don't make SplitPerApiLevel imply UseSdk

UseSdk was returning true when SplitPerApiLevel returned true,
which was causing the platform variant of SplitPerApiLevel
module to compile against the SDK.  Check SplitPerApiLevel
separately in the sdkMutator instead.

Test: m checkbuild
Change-Id: I0ae667d48a3b7b96709a6cad8e8ea9701659fc2a