Revert "Removing gsi_keys.mk"

This reverts commit 5d2cf578.

Reason for revert: DroidMonitor-triggered revert due to breakage https://android-build.googleplex.com/builds/quarterdeck?branch=git_sc-qpr1-dev-plus-aosp&target=build_test3&lkgb=8021524&lkbb=8021634&fkbb=8021569, bug https://buganizer.corp.google.com/issues/211737534

Bug: 211737534
Change-Id: I40e8dce5d87a1ba08a6f100fc2af92dd94fd2d5c