Add small improvement to Manifest import feature docs

Dmytro Zaporozhets (DZ)requested to merge
dz-improve-manifest-doc into master

What does this MR do?

  1. Adds a link to manifest used by AOSP
  2. Mentions both Android and AOSP keyword for better search

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/28811

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