desktop: Wrap-up gomobile JAR branch

[sdsantos]

Source branch: https://github.com/hellais/mobile/tree/plainjava-pr

• Make sure the Android AAR generation is still working
• Figure out if android.content.Context is optional, or if we need different versions of Seq.java` for Android and Java targets.
• Create a PR upstream on https://github.com/golang/mobile

[hellais]

Assigning this to @DecFox. There is some code in the oomobile repo: https://github.com/ooni/oomobile/tree/oomobile, which needs to be reviewed and refined. We should also add support in probe-cli for using our fork allowing us to produce cross-platform builds of the engine to support the desktop applications.