Move monolithic stub flags generation to platform_bootclasspath

As part of that this change:
* Moves code that will be common to platform_bootclasspath and
  bootclasspath_fragment from hiddenapi_singleton.go into
  hiddenapi_modular.go.
* Fixes the tests in hiddenapi_singleton_test.go but intentionally
  does not rename them or move them into a more appropriate place so
  as to make it easier to see the differences. A TODO has been added
  and these will be cleaned up in a follow up change.

Bug: 179354495
Test: verified that the monolithic out/soong/hiddenapi/... files are
      unchanged by this change
Change-Id: I680e4dab2e6bdf4a655fa9f255c195175904667e