[sdk-platform-java] Compiling generated libraries with Java 9+ fails due to javax.annotation.Generated

For example, compiling a "self-service" generated library with `./gradlew check` fails:
```
> Task :proto-google-cloud-retail-v2alpha-java:compileJava
/tmp/google-cloud-retail-v2alpha-java/proto-google-cloud-retail-v2alpha-java/src/main/java/com/google/cloud/retail/v2alpha/LocationName.java:27: error: cannot find symbol
import javax.annotation.Generated;
```

The `javax.annotation` package space is really a mess from the perspective of Java 9+ JPMS, and it has caused a lot of trouble to many people with Java 9+.

The right solution for Java 9+ is to drop `javax.annotation.Generated` and migrate to `javax.annotation.processing.Generated`. However, doing so will cause Java 8 to fail with the same "cannot find symbol" error. An option would be to completely drop using the annotation. However, the gRPC proto compiler generated files with the annotation, so they are out of our control:
```
@javax.annotation.processing.Generated(
value = "by gRPC proto compiler",
comments = "Source: google/cloud/retail/v2alpha/product_service.proto")
```

A workaround for the users hitting this issue is to declare the following dependency, but this isn't really the right solution.
```gradle
implementation 'javax.annotation:javax.annotation-api:1.3.2'
```

Refs:
https://github.com/google/dagger/issues/880
https://github.com/grpc/grpc-java/issues/3633
https://github.com/vavr-io/vavr/issues/2154

首先，在 proto-google-cloud-retail-v2alpha-java 之类的生成库中使用 ./gradlew check 重现失败，然后检查生成的 LocationName.java 的 import 和 gRPC proto 编译器配置。比较 issue 中描述的 Java 8 和 Java 9+ 注解约束。完成的标准是：生成库在受支持的 Java 版本中编译时不需要 javax.annotation-api workaround。

由索引模型根据 Issue 内容生成。