Support hidden API processing for modules that use platform APIs

Previously, hidden API processing could only be done by those
bootclasspath_fragment modules that either did not depend on any other
fragments (e.g. art-bootclasspath-fragment) or only depended on APIs
provided by other fragments (e.g. i18n-bootclasspath-fragment). That
meant that modules like com.android.os.statsd-bootclasspath-fragment
that depended on APIs provided by parts of the platform which are not
yet part of another bootclasspath_fragment could not perform hidden
API processing.

This change adds support for a bootclasspath_fragment to specify the
additional stubs needed to perform hidden API processing. It adds a new
additional_stubs property that can be used to specify the additional
stub libraries.

Most bootclasspath_fragments that need to use the property will need
access to the APIs provided by the android-non-updatable.* libraries.
Rather than have each fragment explicitly specify the correct module
for each scope it treats "android-non-updatable" as if it was a
java_sdk_library that can provide different jars for each scope.
Soong will handle mapping that to the correct android-non-updatable.*
module.

Bug: 179354495
Test: m out/soong/hiddenapi/hiddenapi-flags.csv \
        out/soong/hiddenapi/hiddenapi-index.csv \
        out/soong/hiddenapi/hiddenapi-stub-flags.txt \
        out/soong/hiddenapi/hiddenapi-unsupported.csv
      - make sure that this change does not change the contents.
      m TARGET_BUILD_APPS=Calendar nothing
Change-Id: Ia8b79830ed0e6d42100de03d76b0c51b7f6c8ade