Add a new sdk_version "core_platform_current"

Add a new sdk_version "core_platform_current"
value to java_library.

This adds the ability to compile system code against the
core platform API stubs. These stubs will contain the public SDK
API _and_ a selection of extra methods just intended
for use by system code (e.g. framework .jar) and
which must be preserved if "core" is modularized to
retain source and binary compatibility. Methods
outside of the core platform API must not be relied upon.

Future changes will adding methods to the core platform API and
switch targets over to use the stubs.

As soon as feasible, (hopefully) the default for when
unspecified will be changed to be the same as specifying
core_platform_current and build rules will have to explicitly
specify when they want to compile against the core library
implementation directly.

Bug: 113148576
Test: mmm libcore/mmodules/core_platform_api_client_demo
Change-Id: I72a03f28a4c38b4232e513a088c2d4e962c98868