feat(utils): dismissKeyboard, copyToClipboard, setWindowBackgroundColor, getCurrentActivity and getResource

[NathanWalker]

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/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/blob/master/tools/notes/DevelopmentWorkflow.md#running-unit-tests-application.

What is the current behavior?

Various commonly needed utilities required developers to create themselves.

What is the new behavior?

New convenient utilities have been added:

• Utils.dismissKeyboard(): dismiss any visible keyboard
• Utils.copyToClipboard(value): copy any string value to device clipboard
• Utils.ios.setWindowBackgroundColor('blue'): iOS set window background (shown behind modals)
• Utils.android.getCurrentActivity(): get current activity - previously would often check foregroundActivity and startActivity - this fallbacks automatically
• Utils.android.getResource(name, type?): get an android resource by it's name with optional explicit type. This also uses the explicit package name unlike getId.