Cronet impl (and shared) code

This directory contains the so-called Cronet “impl” code, which provides working production implementations of the Cronet API. Currently this includes:

• The “native” CronetEngine implementation backed by //net, which is what most people refer to as simply “Cronet” and comes with all the bells and whistles. Relevant classes are typically identified by a Cronet prefix;
• The “fallback” or “Java” lightweight implementation backed by java.net.HttpURLConnection. This implementation is much smaller and lacks many features. Relevant classes are typically identified by a Java prefix.

This directory also contains so-called “shared” code, which is shared between all Cronet packages, including the API package.

TODO(crbug.com/40947707): these should really be three distinct directories.

IMPORTANT: in most Cronet release channels (e.g. Google-internal, Maven, AOSP) the API and impl code are expected to always be in sync with each other, i.e. they come from the same build. That is NOT true, however, when loading Cronet from Google Play Services. In that case the app will typically bundle a Cronet API library that is from a different build/version than the Cronet native impl code provided by Google Play Services on a given device. In practice this means that the API code and native impl code must be backwards and forwards compatible with each other at the Java ABI level. Note, though, that Google Play Services only provides native Cronet - this is not an issue for the fallback implementation.