core.current.stubs: add LOCAL_PATCH_MODULE := java.base

The stub classes from core.current.stubs are compiled with libcore
on the bootclasspath / system modules. Since the stub classes are
in the same packages as libcore classes (they carry the same names),
this breaks under OpenJDK 9 javac -target 1.9.

This CL adds LOCAL_PATCH_MODULE := java.base, which means that
javac will be invoked with --patch-module=java.base=., fixing
the compilation.

Bug: 72206056
Bug: 38177569
Test: EXPERIMENTAL_USE_OPENJDK9=true USE_R8=true make checkbuild docs
      in a workspace that also included the following CL topic:
http://r.android.com/q/topic:bug72703434_pin_jarjar_targets_to_java_language_version

Change-Id: Iebaf3693bcc2a3ec99cd56d8fe878412bccceb71