Adds FakeFeatureFlags for testing

No default values are used or provided from this class.
Reading a flag value which has not been previously `set`
will result in an IllegalStateException.

This is designed to prevent tests which unknowingly rely
on default flag values to succeed. By specifying flag
values up front, a test documents dependencies in code.

Test: atest FakeFeatureFlagsTest
Change-Id: Ie367d784a4005be4686875ce5e992ad92155379a