As of Bazel 0.4.5, the android_sdk_for_test and android_ndk_for_test are bound in android.WORKSPACE, so we do not need to include them in the main Bazel WORKSPACE file. I've update the comments to reflect that all that is needed to run the tests is android_sdk_repository and android_ndk_repository and the environment variables that they read.

Also, delete scripts/workspace_user.sh. All of its functionality (reading environment variables, detecting api levels, build tools versions) is now part of android_{s,n}dk_repository.

Fixes https://github.com/bazelbuild/bazel/issues/2284.

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PiperOrigin-RevId: 151032551
MOS_MIGRATED_REVID=151032551