docs: upgraded version of xam forms and changed ondestroy to use flush instead of shutdown

[PureWeen]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Bug fix and upgraded xamarin forms version

What is the current behavior? (You can also link to an open issue here)
Currently in OnDestroy on the MainActivity it was calling BlobCache.Shutdown which completely shuts down the cache from ever being used again. In cases where the MainActivity is destroyed but static things stay in memory this caused the app to stop working.

you could recreate by

• app started
• click back button to minimize app (this destroys the main activity)
• start app back up and nothing renders

What is the new behavior (if this is a feature change)?

 protected override void OnDestroy()
        {
            BlobCache.LocalMachine.Flush().Wait();
            base.OnDestroy();
        }

Here's a longer discussion about this
reactiveui/Akavache#342

As an example it'd probably be good to just not use LocalMAchine all together but this works for now

What might this PR break?

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

[dnfclas]

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[coveralls]

Changes Unknown when pulling 1b86b94 on PureWeen:upgrade_xam_forms_change_cache_to_flush_from_shutdown into ** on reactiveui:develop**.

[ghuntley]

Thank-you @PureWeen love your work!