Generate hidden API flags for a bootclasspath_fragment

This change adds support for generating the hidden API flags for the
contents of a bootclasspath_fragment. Currently, it will only work for
the art-bootclasspath-fragment as it has no support for creating
dependencies between bootclasspath_fragment modules which will be
needed for handling any other bootclasspath_fragment.

The hidden API flag generation added by this change is completely
separate to the normal hidden API processing and is not as yet encoded
in dex jars so will have no effect on the runtime.

The generated files are provided for use by other modules and copied
into the sdk snapshot. That is needed to allow the build to verify that
the hidden API flags generated by the individual bootclasspath_fragment
modules are consistent with the flags generated for the whole
bootclasspath, whether building from source or prebuilts.

Bug: 179354495
Test: m art-module-sdk
      m out/soong/.intermediates/art/build/boot/art-bootclasspath-fragment/android_common_apex10000/modular-hiddenapi/all-flags.csv
      m out/soong/hiddenapi/hiddenapi-flags.csv
      - test that the former file is a subset of the latter and that
        where they overlap they are identical.
Change-Id: Ie27303e2960953db1b7abe95510e3bca4411b09a