Update Android-related tests to use Android Platforms flags

A few test targets in this project are using either `android_binary` or `aar_import` targets, and so need to be updated to work with Android platforms (https://github.com/bazelbuild/bazel/issues/16285).

Test `//tests/unit/aar_import/...` is relatively easy: we need to define a `platform` with the android OS constraint and use that with the `--platforms` flag (because these are not part of an actual `android_binary`, we cannot use `--android_platforms`.

Test `//tests/integration/override_targets/...` is harder: when I did the above (using `--android_platforms` because this _is_ an `android_binary`), I got very confusing errors due to the `override_targets` feature that is being tested. It looked from the errors as if the `//tests/integration/override_targets:sceneform_rendering` target is being configured both as a dependency of `:app` (which is correct), and in the top-level configuration (which is wrong, and causes build errors because the target platform is wrong).

I don't understand the override targets feature but this feels like a bug to me.

For now I am sending a PR to disable Android platforms for these tests, but this should be addressed.

Start with the targets under //tests/unit/aar_import/... and define the Android platform used with --platforms. Then investigate //tests/integration/override_targets/... and the sceneform_rendering configuration with --android_platforms, focusing on why the target appears in both dependency and top-level configurations. Done means both test areas work with Android platforms without the confusing configuration errors.

Written by the indexing model from the issue text.