Add hook to be called after defaults have been applied

Previously, the only way for a module type to create modules (other
than defining its own mutator) was to register a LoadHook. However,
that is called before defaults are applied so any properties used in
that hook cannot take advantage of defaults, e.g. java_sdk_library
cannot use defaults to set its sdk_version property and have that
affect its child modules.

This change adds a new SetDefaultableHook() to DefaultableModule to
register a hook that is called after any defaults have been applied.

Also adds some tests to ensure that errors in the visibility property
introduced in a DefaultableHook are reported in the gather phase of
visibility processing.

A follow up change will switch java_sdk_library to use that instead
of the AddLoadHook() mechanism.

Bug: 155295806
Test: m checkapi
Change-Id: I13df3115f9e225f7324b6725eaeb16a78cc2538a

android/hooks.go

@@ -39,6 +39,12 @@ type LoadHookContext interface {
 	moduleFactories() map[string]blueprint.ModuleFactory
 }
 
+// Add a hook that will be called once the module has been loaded, i.e. its
+// properties have been initialized from the Android.bp file.
+//
+// Consider using SetDefaultableHook to register a hook for any module that implements
+// DefaultableModule as the hook is called after any defaults have been applied to the
+// module which could reduce duplication and make it easier to use.
 func AddLoadHook(m blueprint.Module, hook func(LoadHookContext)) {
 	blueprint.AddLoadHook(m, func(ctx blueprint.LoadHookContext) {
 		actx := &loadHookContext{