cronet_http: require android API level 28

[brianquinlan]

• Ensure that SDK version requirements are consistent (and document to change them in sync)
• Ensure that embedded cronet does not use Google Play Services
• Minor documentation updates

My intent is to publish package:cronet_http_embedded 0.4.2 from this source. I'll then land #1087 and publish package:cronet_http.

So, at that point, package:cronet_http and package:cronet_http_embedded will be functionally identical but package:cronet_http_embedded will still have a <1.0 version number.

In the new year we can sort out what publisher to use for package:cronet_http_embedded.

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