Allow setting api_level via --action_env

[tawakasdf]

At the moment the api_level parameter can only be set via attrs inside WORKSPACE, which will be removed soon. Or through android_ndk_repository_extension = use_extension(
"@rules_android_ndk//:extension.bzl",
"android_ndk_repository_extension",
)

android_ndk_repository_extension.configure(api_level = 21)

use_repo(android_ndk_repository_extension, "androidndk"), which is not flexible

[ted-xie]

I would much rather this be done via a starlark flag or even a --define argument. --action_env gets propagated to all actions IIRC, and would invalidate all actions in your cache.