Download the NDK in Bazel

[gferon]

The rule now ignores ANDROID_NDK_HOME and defaults to downloading the r25c NDK for the current platform (failing if the platform is not supported).

One can specify three new attributes:

• version to select the NDK to download
• base_url if you don't want to use https://dl.google.com/android/repository
• sha256sums to add checksums for (yet) unsupported NDK releases

Also:

• Specify the template files labels in repository_ctx to avoid the rule implementation function from being restarted (this wasn't so visible with the local install)
• Use default in attribute for api_version
• Remove the call to native.register_toolchains() since this will register the NDK as the default C++ toolchain, which shouldn't be the default assumption.

TODO:

• Provide labels via ctx attributes to avoid the rule restarting multiple times

Closes #44

[gferon]

This is ready to review and 🚀 - we're using it as-is in our workspace and everything works nicely.

[ahumesky]

Thanks, I'm asking internally whether we can have such a feature in rules_android, since the download pages for the NDK (and SDK fwiw) present Terms and Conditions that you have to accept before downloading.

[gferon]

Fair point, I guess that could be a problem. However, if this is required, we could maybe think of introducing a feature that users need to toggle once per host or something like that?

EDIT: another idea is to add a flag something_read_eula and set it to False in the examples.

[gferon]

One more data point if that helps: I just noticed that the flutter command downloads Android SDK modules, and accepts licenses automatically by default.

[marcushultman]

Any chance this will land anytime soon? This seem to close #2 too.