So as part of
envoyproxy/envoy-mobile#2003 we need to fix the runtime singleton problem.

Actual runtime is no problem but having runtimeFeatureEnabled point at an actual runtime is basically impossible to fix.

I propose replacing the runtime guards with ABSL flags: still setting them via RTDS but being able to access them directly as flags. This solves both a bunch of test issues where flags were accessed before/after the envoy server comes up and down, and allows for removing the singleton issue for envoy mobile.

E-M won't support different values for runtime across different instances, but we can't do that without wiring some server-specific handle to all the locations where we use runtime which would make adding new values prohibitively cumbersome anyway.