documentation for targets under android_sdk_repository

there are a number of useful targets under an instance of `android_sdk_repository` that are not documented, e.g.
```
@androidsdk//:legacy_test-28
@androidsdk//:legacy_test-29
@androidsdk//:legacy_test-30
@androidsdk//:legacy_test-31
@androidsdk//:legacy_test-32
@androidsdk//:legacy_test-33
@androidsdk//:legacy_test-34
```
i didn't know these existed until i ran `bzl query @androidsdk//...` - could we add some documentation for these targets?