maven_export support for bundling AARs

[sugarmanz]

This PR adds (limited) support for bundling AARs for distribution with maven_publish. It optimistically uses an .aar from AndroidLibraryAarInfo as the basis for bundling and then merges the transitive closure with the nested classes.jar using the existing logic.

Usage of AndroidLibraryAarInfo requires an --experimental_google_legacy_api flag, which is pretty undesirable given it would require consumers to configure the same flag, even if not using for publishing Android artifacts. As such, I understand this PR probably isn't acceptable from a generic standpoint, but I wanted to open it up for feedback, ideas, and inspiration. This is what we'll be using to publish our Android targets until we can find a better alternative.

Example usages:

kt_jvm_library(
    name = "kt-lib",
    srcs = glob(["src/main/java/**/*.kt"]),
    resources = glob(["src/main/resources/**"]),
    exports = main_exports,
    deps = main_deps,
)

android_library(
    name = "android-lib",
    manifest = ":src/main/AndroidManifest.xml",
    resource_files = glob(["src/main/res/**"]),
    tags = ["maven_coordinates=group:artifact:aar:%s" % VERSION],
    exports = main_exports + [
        ":kt-lib",
    ],
)

maven_export(
    name = "export",
    lib_name = "android-lib",
    maven_coordinates = "group:artifact:aar:%s" % VERSION,
)

External example (we're using an Android databinding wrapper which does work to create a base .aar instead of android_library):
https://github.com/player-ui/player/blob/78345ac70c5bb34fcd58147ab49e38f22d3524fd/android/player/BUILD#L10-L27

Semi-relevant:
bazelbuild/bazel#348

[sugarmanz]

Relevant discussion for this change:
vaticle/bazel-distribution#318

Might break this out into a separate contribution since this is unrelated, yet necessary for publishing support for Artifactory repositories.

[jin]

@ahumesky could you advise on the use of --experimental_google_legacy_api and AndroidLibraryAarInfo for this?

Or are there alternative upstream mechanisms that tackles bazelbuild/bazel#348?