Update the CI to use Android NDK r25b

[chriswailes]

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[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Amanieu (or someone else) soon.

Please see the contribution instructions for more information.

[JohnTitor]

Thanks for the PR! I've, however, updated the toolchain on #2955, closing in favor of that.

[chriswailes]

A couple of comments on #2955:

1. The use of make_standalone_toolchain.py is no longer recommended and using it with the compiler wrappers is explicitly not supported. See https://developer.android.com/ndk/guides/other_build_systems
2. The Android Platform Support document says that rustc should target the lowest level API supported by a target for a given NDK unless there is a specific reason a higher API level is needed. If libc needs to use API level 28 we should update the rustc CI and document it.
3. Updates to aarch-linux-android/Dockerfile, arm-linux-androideabi/Dockerfile, and x86_64-linux-android/Dockerfile are missing.