Previously, the hidden API encoding of the tethering boot dex jars,
i.e. those dex jars that tethering contributes to the bootclasspath
were done as part of the monolithic hidden API processing. This change
causes the encoding to be done by the tethering's
bootclasspath_fragment.

This change involves the following:
* Addition of the fragments property to the tethering's
  bootclasspath_fragment module to list all the other
  bootclasspath_fragment modules on which this depends.
* Addition of the additional_stubs property to add stubs for APIs that
  are not provided by another bootclasspath_fragment.
* Moving hidden API flag file entries related to tethering from the
  flag files in frameworks/base/boot/hiddenapi directory into the
  tethering directory with an appropriate OWNERS file to allow them to
  be managed by the Soong and compat team.
* Addition of a PREUPLOAD.cfg hook script to ensure that the flag files
  are sorted.

The build automatically checks that the hidden API flags which are
computed by tethering and encoded into its boot dex jars match those
that are generated by the monolithic processing so this is guaranteed
to be safe.

Bug: 179354495
Test: m com.android.tethering
      - ensure that the generated APEX is byte-for-byte identical
        before and after these changes.
      m out/soong/hiddenapi/hiddenapi-flags.csv
      - make sure that they are not changed by this.
Change-Id: I4d9621325c7fcea5043cbca4c577ba2ac6125c0c
Ignore-AOSP-First: merge conflicts