Allow hiddenapi list to ignore missing classes/members

When generating the stub-flags.csv for a bootclasspath_fragment the
hiddenapi list tool is not given a complete set of all classes and
members. This change causes it to ignore them by passing the new
--fragment option to it.

This does not risk changing the flags as the stub-flags.csv files
created with the --fragment option are compared with the monolithic
out/soong/hiddenapi/hiddenapi-stub-flags.txt file which is not run
with this option to ensure that they match.

Bug: 179354495
Test: m out/soong/hiddenapi-stub-flags.csv
      - make sure that this does not change the file.
Change-Id: I890c7374c445759cade4d685f51e81261b7ccea2

java/hiddenapi_modular.go

@@ -316,6 +316,13 @@ func buildRuleToGenerateHiddenAPIStubFlagsFile(ctx android.BuilderContext, name,
 		FlagForEachInput("--dependency-stub-dex=", dependencyStubDexJars).
 		FlagForEachInput("--boot-dex=", bootDexJars)
 
+	// If no module stub flags paths are provided then this must be being called for a
+	// bootclasspath_fragment and not the whole platform_bootclasspath.
+	if moduleStubFlagsPaths == nil {
+		// This is being run on a fragment of the bootclasspath.
+		command.Flag("--fragment")
+	}
+
 	// Iterate over the api scopes in a fixed order.
 	for _, apiScope := range hiddenAPIFlagScopes {
 		// Merge in the stub dex jar paths for this api scope from the fragments on which it depends.