TB 40002: [android] Ensure system download manager is not used

Originally, android-components#40002.

android-components#40075: Support scoped storage to enable downloads on API < 29

- in android-components!7,  we blocked all usage of Scoped
  Storage in an attempt to block usage of Android's
  DownloadManager, which is known to cause proxy bypasses
- as of Android API 29, downloads will not work without Scoped Storage,
  causing all downlaods to fail (see: fenix##40192)
- here, we enable usage of scoped storage for API >= 29, but block
  calls to DownloadManager on API < 29