add Security First Umbrella via reproducible build process

Security First Umbrella is a free software app for security training. It should be added via the reproducible build process. Upstream is fully supportive of this effort.

• https://github.com/securityfirst/Umbrella_android
• https://play.google.com/store/apps/details?id=org.secfirst.umbrella

pinging @lamby since I can't assign