Build against SDKs for native libs only for unbundled apps.

Unbundled APEX modules need to access the platform variant (i.e.
sdk:"") since it's there that the versioned stubs are exposed.

Test: m SOONG_ALLOW_MISSING_DEPENDENCIES=true \
        TARGET_BUILD_UNBUNDLED=true toybox
  on master-art with prebuilt Runtime (Bionic) APEX SDK. The prebuilt
  SDK doesn't have libc.ndk.xxx etc, which e.g. libcrypto would depend
  on without this CL, since it specifies sdk_version.
Bug: 157549171
Change-Id: I3095e42beb2b48421bfb81be942cc2ac30405fd0