Add a simple fake implementation of Icing to unblock development.

This class is in-memory only. It must be replaced with icing-lib before the feature can be used in production.

Test: atest --rebuild-module-info FrameworksServicesTests:FakeIcingTest
Bug: 143900788
Change-Id: I3cf30b9c8b429b3ba44b3c04379bf86b291b02f9