MXKEncryptionKeysImportView improvement

`MXKEncryptionKeysImportView:showInViewController:toImportKeys:onComplete` needs the following improvements:
- The first param must be `UIViewController ` instead of `MXKViewController`
- The `onComplete` block should return the error (nullable) to let the application handle it

Start by locating the declaration of MXKEncryptionKeysImportView:showInViewController:toImportKeys:onComplete and its call sites. Review how the completion block is currently typed and invoked, then update the affected callers and verify the project builds with the protocol-based controller parameter and nullable error result.

Written by the indexing model from the issue text.