Support hidden API processing for fragments with dependencies

Previously, a bootclasspath_fragment that depended on classes provided
by another bootclasspath_fragment did not support hidden API processing
as it would not supply information about those dependencies.

This change adds support for that as follows. Each fragment:
1. Exports the transitive sets of stub dex jars for each of the public,
   system, test and core_platform APIs (where relevant).
2. Adds dependencies onto its dependent fragments.
3. Retrieves the API stubs dex jars from its dependent fragments and
   passes them to the "hiddenapi list" tool which will use them to
   resolve dependencies but will not output them to the generated
   flags.

Once the flags are generated the existing encoding functionality
encodes the flags into the dex files of the bootclasspath_fragment's
content modules which are then packaged into the apex.

Bug: 179354495
Test: m com.android.sdkext
      - verify that this does not change the contents of the apex files
Change-Id: I3e82a6dbb437f1e417e5d7e25aeb212e378603d0