ApiLevel of "" and "core_platform" is FutureApiLevel

If sdk_version is set to "", it means the module is built with the
in-development version of the platform APIs. "core_platform" means the
in-development version of the core Java APIs. In both cases, the API
level (i.e. which version to use) is the in-development version.

Bug:  1663140
Test: m

Change-Id: Ia184190341223e9ac12710a8bb3a25004fd4f539