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Wrap Cronet callback/listener classes in wrapper classes whose job it
is to enforce that callbacks added in later versions are not called
when the client API is of an older version.  For now all wrapper
classes simply pass through all the callbacks as we're still working
on the first version of the API, which we shall support indefinitly.

A side benefit is this facilitates only passing implementation, not
API, classes to JNI functions like nativeGetStatus.  This is
required to support cases where the Cronet API may be repackaged
into another class at the Java layer, but not at the JNI layer.

CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_cronet_tester
BUG=629299
R=kapishnikov

Review-Url: https://codereview.chromium.org/2514783002
Cr-Commit-Position: refs/heads/master@{#434891}