documentation for targets under android_sdk_repository

[master-hax]

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?

[master-hax]

cc @ahumesky