Define and implement a service for key management in CompOS.

This is based on the interface prototyped in
system/security/ondevice-signing/FakeCompOs.h.

For now, to allow manual testing, this is a standalone binary.

Bug: 191763370
Test: Builds.
Change-Id: I307ba9144fa51cca7ebee2142980f3a1cd436ef2
6 files changed