Send HEADERS_RECEIVED and LOADING events on Android

Summary: Send part of the response body every 100 ms if the client has set onreadystatechange. This
is done by using the same events as the iOS code and removing the callback that Android previously
used.

Reconsolidate iOS and Android implementations.

Closes #3772

public

Reviewed By: mkonicek

Differential Revision: D2647005

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