Tidy up API build variables

1) Delete unused Docs.mk.

This removes the last reference to non_openjdk_java_files.bp and
openjdk_java_files.bp filegroups from .mk files.
This means that export_to_make_var can be removed in those files.

2) Introduce a filegroup, core_public_api_files, that can be used
from frameworks/base/ rather than "internal" variables defined in
non_openjdk_java_files.bp, openjdk_java_files.bp and conscrypt.

Bug: 126332291
Test: build only
Change-Id: I3c85d17dd7405e65a6be9e60831b1a07e4f08abf