fragment下监听onActivityForResult

[wangrunxiang]

增加方法，实现Fragment.startActivityForResult，用来方便在fragment下监听onActivityForResult

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[zhi1ong]

感谢贡献。这个 PR 当前不能合入：startActivity 中对 fragment != null 和 currentContext instanceof Activity 使用了两个独立的 if。Fragment 调用时通常两个条件都成立，会先调用 Fragment.startActivityForResult，再调用 ActivityCompat.startActivityForResult，导致目标 Activity 被启动两次。此外，新 API 固定依赖 android.support.v4.app.Fragment 和已弃用的 onActivityResult 模型，不符合后续 AndroidX / Activity Result 的兼容方向。相关能力由 #905 统一跟踪；关闭此 PR，后续请基于最新 develop 和 #905 的 API 方案重新提交。