This allows integrating an Android test project easily.

Simplified RxJavaCallAdapterFactory.createAsync documentation to not mention any incorrect subscribeOn behavior

| Library            | From     | To        |
| :----------------- | :------- | :-------- |
| animal.sniffer     |  1.14    |  1.18     |
| assertj            |  1.7.0   |  3.13.2   |
| guava              |  19.0    |  28.1-jre |
| jackson            |  2.9.8   |  2.9.10   |
| jaxb               |  2.2.12  |  2.3.1    |
| jsoup              |  1.7.3   |  1.12.1   |
| kotlin             |  1.3.10  |  1.3.50   |
| kotlinx.coroutines |  1.1.0   |  1.3.2    |
| okhttp             |  3.14.0  |  3.14.4   |
| protobuf           |  3.5.0   |  3.10.0   |
| rxjava             |  1.3.0   |  1.3.8    |
| scala              |  2.12.4  |  2.13.1   |

The XML format changed again, so I decided to switch us to Checksytle's
curated 'Google Style Guide' format which seems like a reasonable
enough default for us.

https://checkstyle.sourceforge.io/google_style.html

Also fix some Javadoc errors and ensure Javadoc (and source) jar generation is tested on CI.

This is an imperfect fix as uncaught exceptions reaching OkHttpCall which are not fatal still only log, but most of the adapters are defending against anything getting this far already.

Also contained is a fix for the RxJava 2 async adapter. Non-200 responses delivered to the body observer would be forwarded to onError. If that callback threw a fatal exception, it would be re-delivered to the body observer despite it having already called a downstream terminal method. The fatal error should have been propagated by the async observer instead of attempting to make its way back downstream.

Use get instead of parse for HttpUrl and MediaType where valid input is always required.

This allows things like error and trace tracking to work naturally.

Fixes #2716