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

java/bootclasspath_fragment.go

@@ -949,7 +949,7 @@ func (b *bootclasspathFragmentSdkMemberProperties) AddToPropertySet(ctx android.
 					builder.CopyToSnapshot(p, dest)
 					dests = append(dests, dest)
 				}
-				hiddenAPISet.AddProperty(category.PropertyName, dests)
+				hiddenAPISet.AddProperty(category.PropertyName(), dests)
 			}
 		}
 	}