Support sdk_version: "core_current"

core_current is a pseudo SDK version which is a core Java API subset of
the Android API. It is expected to be mainly used for external Java
projects which are agnostic to Android; such as junit, guava, etc.

A module built with this SDK version can only link to java modules of
the same kind. It can't depend on modules built with other sdk  (e.g.
current) or without sdk.

Bug: 72206056
Test: m -j
Change-Id: I778e7b4fcb9456a12b418ffd633ea78e29951e84