platform_bootclasspath: aggregate hidden API flag files from fragments

Aggregates hidden API flag files from the bootclasspath_fragments which
will allow the hidden API flag files in frameworks/base/boot/hiddenapi
to be modularized and moved to the appropriate repo.

Bug: 177892522
Test: verified that the out/soong/hiddenapi/... files are unchanged
      by this change
      also verified that changes to the fragment provided files do
      affect the monolithic files.
Change-Id: Ifce14c9ef24c58c7ab1085475d85b61cfbfefecd