feat: 对齐 Android 模板目录结构

[dream93]

将 Android 模板工程结构对齐 Cocos 官方默认模板：

• app / instantapp 的 AndroidManifest.xml 移动到 src/main/ 下
• AppActivity.java / InstantActivity.java 移动到 src/main/java/ 下
• build.gradle 的 sourceSets 相应调整，新增 src/main/res、src/main/assets 目录

好处：

• 多渠道打包可在 src 下建立对应目录进行配置，无需手动设置 sourceSets
• 对齐 SDK 往 assets 目录添加文件的标准路径，降低接入复杂度，同时保持向后兼容

Re: #

Changelog

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Continuous Integration

This pull request:

• needs automatic test cases check.

  Manual trigger with @cocos-robot run test cases afterward.

• does not change any runtime related code or build configuration

  If any reviewer thinks the CI checks are needed, please uncheck this option, then close and reopen the issue.

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Compatibility Check

This pull request:

• changes public API, and have ensured backward compatibility with deprecated features.
• affects platform compatibility, e.g. system version, browser version, platform sdk version, platform toolchain, language version, hardware compatibility etc.
• affects file structure of the build package or build configuration which requires user project upgrade.
• introduces breaking changes, please list all changes, affected features and the scope of violation.