Extract ruleToGenerateHiddenApiFlags

Extracts the code for creating the rule that creates the monolithic
hidden API flags file which is encoded into dex implementation jars.
This refactoring is in preparation for moving the functionality from
the hiddenapi_singleton into the platform_bootclasspath.

A follow up change will move the method into the new
hiddenapi_modular.go alongside the hiddenAPIAugmentationInfo as they
will both be used to perform hidden API processing for a
bootclasspath_fragment.

Bug: 177892522
Test: verified that the out/soong/hiddenapi/... files are unchanged
      by this change
Change-Id: I2729afa80cdfd2d1d4717365001648453d65632f