Move generation of global hidden API flags to platform_bootclasspath

This change moves the generation of the global hidden API flags from
the singleton to the platform_bootclasspath module. It involves:
1. Moving the ruleToGenerateHiddenApiFlags to hiddenapi_modular.go.
2. Adding HiddenAPIAugmentationProperties to be used by the
   platform_bootclasspath type.
3. Moving the file paths into the platform-bootclasspath module
   definition in frameworks/base/boot/Android.bp.

The flagsRule is kept as a placeholder for now. The emptyFlagsRule is
also kept so that builds continue to work even when the frameworks/base
repository is not present.

Bug: 177892522
Test: verified that the out/soong/hiddenapi/... files are unchanged
      by this change
Change-Id: Idf4dd414a016831bfe04a01f93234c1c33819881