Allow pruning of unsupported fields in structs in maps

Adds support for traversing into a field that is of type:
   map[...]*struct{...}

This is needed to allow java_sdk_library to mark scope specific
properties, e.g. public.annotations as being target build release
specific.

It was necessary to change the Scope field from:
   Scope map[*apiScope]scopeProperties
to:
   Scope map[*apiScope]*scopeProperties

That is because there is no way in go to change the field of a struct
value of a map. i.e. you cannot do the following, not even using
reflection:
   Scope[apiScopePublic].AnnotationsZip = nil

Bug: 204763318
Test: m nothing
Change-Id: Id103f70f55d4202971321ef4925cbec4b55f8136

java/sdk_library.go

@@ -2755,7 +2755,7 @@ type sdkLibrarySdkMemberProperties struct {
 	android.SdkMemberPropertiesBase
 
 	// Scope to per scope properties.
-	Scopes map[*apiScope]scopeProperties
+	Scopes map[*apiScope]*scopeProperties
 
 	// The Java stubs source files.
 	Stub_srcs []string
@@ -2815,7 +2815,7 @@ type scopeProperties struct {
 func (s *sdkLibrarySdkMemberProperties) PopulateFromVariant(ctx android.SdkMemberContext, variant android.Module) {
 	sdk := variant.(*SdkLibrary)
 
-	s.Scopes = make(map[*apiScope]scopeProperties)
+	s.Scopes = make(map[*apiScope]*scopeProperties)
 	for _, apiScope := range allApiScopes {
 		paths := sdk.findScopePaths(apiScope)
 		if paths == nil {
@@ -2838,7 +2838,7 @@ func (s *sdkLibrarySdkMemberProperties) PopulateFromVariant(ctx android.SdkMembe
 			if paths.annotationsZip.Valid() {
 				properties.AnnotationsZip = paths.annotationsZip.Path()
 			}
-			s.Scopes[apiScope] = properties
+			s.Scopes[apiScope] = &properties
 		}
 	}