Make hiddenAPIFlagFileCategory an int

hiddenAPIFlagFileCategory used to contain function pointers, and is
used in a provider. Providers must be serializable for incremental
soong support, so the function pointers must be removed.

Refactor hiddenAPIFlagFileCategory into a simple int.

Bug: 322069292
Test: m nothing --no-skip-soong-tests
Change-Id: I2bd50fa1b59979f30869b405314cbef16ee345f1