Separate hidden API flag generation from encoding

Previously, the rules for encoding hidden API flags into dex files were
generated in the hiddenAPIRulesForBootclasspathFragment alongside the
rules for creating the hidden API flag files.

This change separates the encoding from the generation of the flags.
This will allow a follow up change to generate API flags for the sdk
snapshot separately without affecting the encoded dex files.

Bug: 240406019
Test: m nothing
Change-Id: I3ab3ff5780e7b2e322665d3ea15f1438f2e403bd