Clear all outletKeys instead of only the first

[karsie]

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
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• All existing tests are passing: https://github.com/NativeScript/nativescript-angular/blob/master/DevelopmentWorkflow.md#running-the-tests
• Tests for the changes are included.

What is the current behavior?

Only the first outletKey of the outlet is used to destroy past views.

What is the new behavior?

All outletKeys of the outlet is used to destroy past views.

Fixes #2303.

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