Disable prebuilt apps without an apk later

Currently, android_app_import modules get disabled during their load
hook if they don't have a set `apk` property. This causes them to be
disabled before soong config variables can be applied, which would've
set the apk property.

Move the disabling into a DefaultableHook, which will run after the
soong config variables.

Bug: 319897584
Test: m nothing --no-skip-soong-tests
Change-Id: Ia0f6a39c35b3b11249bfa74ad532858189be24b1

android/prebuilt.go

@@ -277,6 +277,9 @@ func InitSingleSourcePrebuiltModule(module PrebuiltInterface, srcProps interface
 		}
 		value := srcPropsValue.FieldByIndex(srcFieldIndex)
 		if value.Kind() == reflect.Ptr {
+			if value.IsNil() {
+				return nil
+			}
 			value = value.Elem()
 		}
 		if value.Kind() != reflect.String {