Share Cronet telemetry code between API and impl

IMPORTANT: this CL has breaking implications for downstream Cronet
release channels and requires dependency structure adjustments for
consumers (e.g. internal Google repo, Maven). The new "shared" JAR will
need to be shipped to consumers as a dependency of *both* API and impl
JARs.

This CL introduces a new Cronet distribution JAR,
"cronet_shared_java.jar", which holds code that is shared between all
Cronet code, i.e. the API *and* implementations. Such code is reusable
from anywhere inside the Cronet codebase.

The main distinction between "API" code and "shared" code is that
implementations do not necessarily ship with the API code. The textbook
example of that is the Cronet impl in Google Play Services, which uses
the API classes from the calling app instead of having its own copy.
In contrast, implementations always ship with shared code, such that if
the API code and impl code are loaded from different sources (which
implies potentially different build/version), each side will end up with
its own private copy of the shared classes, thus sidestepping any ABI
compatibility concerns. The downside is that references to these classes
can't cross the Cronet API/impl ABI boundary, but that's fine for what
we want to use this for.

Bug: b:313418339
Change-Id: I6d3293cdb41f4182c005c7176ba0ac49ce053c6d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5094199
Reviewed-by: Stefano Duo <stefanoduo@google.com>
Commit-Queue: Etienne Dechamps <edechamps@google.com>
Cr-Commit-Position: refs/heads/main@{#1259759}

• components/cronet/android/BUILD.gn[diff]
• components/cronet/android/api/README.md[Added - diff]
• components/cronet/android/cronet_combined_impl_native_proguard_golden.cfg[diff]
• components/cronet/android/cronet_impl_common_proguard.cfg[diff]
• components/cronet/android/cronet_shared_proguard.cfg[Added - diff]
• components/cronet/android/java/README.md[Added - diff]