Move generation of hidden API make vars to platform_bootclasspath

Bug: 179354495
Test: rm out/soong/make_vars*
      m nothing
      grep INTERNAL_PLATFORM_HIDDENAPI_FLAGS out/soong/make_vars*
      - make sure it is still out/soong/hiddenapi/hiddenapi-flags.csv
Change-Id: I86e56512a9a2091f446bad25294d41ea1f4341ee