Pass API surface specific extension jars to Metalava

Previously, it would only pass `public` extension jars. This change
fixes that so it passes the jars for the API surface being generated.

Bug: 336993217
Test: m out/target/common/obj/PACKAGING/api_versions_public_generated-api-versions.xml \
        out/target/common/obj/PACKAGING/api_versions_system_generated-api-versions.xml
      # Compare the following files to make sure that they include public extension
      # APIs and system extension APIs respectively.
      diff \
          out/soong/.intermediates/frameworks/base/api/api_versions_public/android_common/*/metalava_exportable.sbox.textproto \
          out/soong/.intermediates/frameworks/base/api/api_versions_system/android_common/*/metalava_exportable.sbox.textproto
Change-Id: I02e2ec3c8176cfbc3cbd8ac56fb78d12b765eedb