fixed "no private key available" issue. Part 1. import key (or load from backups)

[DenBond7]

This PR fixed "no public key available" issue

issue #597

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Tests (delete all except exactly one):

• Tests added or updated

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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

[DenBond7]

How to test:

1. Setup the app for example for [email protected]
2. Go to Settings -> Keys
3. Import a new key for example for [email protected]
4. Delete the key for [email protected]
5. Go to the compose screen and try to send an encrypted message

[sosnovsky]

Works well now 👍

But probably PR name should "no private key available" issue (not public key)