issue #554 add pub keys refresh for contacts

[sosnovsky]

This PR adds autorefresh for local pub keys.

close #554

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Tests (delete all except exactly one):

• Difficult to test (explain why) - LocalContactsProvider uses Realm, we're currently don't have setup for testing Realm models

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To be filled by reviewers

I have reviewed that this PR... (tick whichever items you personally focused on during this review):

• addresses the issue it closes (if any)
• code is readable and understandable
• is accompanied with tests, or tests are not needed
• is free of vulnerabilities
• is documented clearly and usefully, or doesn't need documentation

[sosnovsky]

@tomholub should we start adding tests for Realm models and services to improve test coverage?

[tomholub]

I've removed a few private methods to make it clearer. Looks good, will test.

[tomholub]

@tomholub should we start adding tests for Realm models and services to improve test coverage?

I'm not too worried about testing individual components - except when they do something complicated (what our db does is pretty simple).

I would rather test larger functionality, to make sure it delivers business value. For example, to test that public keys indeed stay up to date. That's a harder test to write, but more valuable. We'll need a bunch of infrastructure for testing that, that I'll bring over from the browser extension.

[tomholub]

Works well. I tested the following scenario:

1. loaded demo@flowcrypt.com in compose
2. checked Contacts in app
3. updated EKM to change expiration of the key
4. loaded demo@flowcrypt.com in compose
5. checked contacts - correctly showed new expiration 👍
6. went to EKM to add one more key to demo@flowcrypt.com user
7. went to compose again and added demo@flowcrypt.com
8. went to contacts - now showing two keys for demo@flowcrypt.com 👍

We'll write a UI test for this later. For the UI test to be viable, the WKD has to be mocked.