fix(android-request): Set custom user-agent from headers on Android request

[phdd]

PR Checklist

• [x] 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.
• [x] All existing tests are passing
• [ ] Tests for the changes are included

What is the current behavior?

The User-Agent header field is ignored on Android.

What is the new behavior?

The method request.setCustomUserAgent gets invoked copying the value from its correspondending header field.

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