Specify more exact assets_dir

[lukaciko]

We saw an issue related to bazelbuild/rules_android#310: files besides crashlytics-build.properties end up affecting and being pulled into outputs of resource processing actions (e.g. assets.bin). We occasionally saw failures like:

ERROR: <foo>/app/debug/crashlytics/BUILD.bazel:11:28: Merging Android assets for //<foo>/app/debug/crashlytics:crashlytics failed: Worker process did not return a WorkResponse:

---8<---8<--- Start of log, file at /private/var/tmp/_bazel_spotify-buildagent/985b781559c7830232d18af8d0be54d9/bazel-workers/worker-12-AndroidAssetMerger.log ---8<---8<---
(empty)
---8<---8<--- End of log ---8<---8<---

which went away if we changed the build_id.

This change generates an assets subfolder with the properties file and specifies that as assets_dir. This way only that single file ends up as an input to the resource processing actions.

[ahumesky]

thank you!