Move global hidden API flags generation to platform_bootclasspath

The global hidden API flags generation has moved from the singleton to
the platform_bootclasspath module type so this change provides the
paths to the additional hidden API flag configuration as part of the
definition of the platform-bootclasspath module.

Bug: 177892522
Test: verified that the out/soong/hiddenapi/... files are unchanged
      by this change
Change-Id: Id82b45d2abd6ca2efcfd730689b1eda92d55978e