Add `gki_retrofitting_tools.zip` to GSI/GKI DIST_DIR

For easy pickup from ci.android.com

The zip archive would contain the `retrofit_gki` tool and its
dependencies plus a README file.

Bug: 210367929
Test: m dist && unzip out/dist/gki_retrofitting_tools.zip
Change-Id: I2ffb457e8c09091e7aaf248b1e18f4c4d96e4492
3 files changed