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.
• You have signed the CLA.
• All existing tests are passing: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/DevelopmentWorkflow.md#running-unit-tests-application.
• Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/tools/notes/WritingUnitTests.md.

What is the current behavior?

Right now, typography is a bit inconsistent between the two platforms and MDN specs.
More specifically, line-height does not act as a height and its visual result is quite different than that of browser.

What is the new behavior?

• Small android size defaults cleanup
• Added support for line-height dip and percentage units and updated calculation to be closer to that of browser
• Corrected UIButton letter-spacing calculation which used the deprecated font property to read font size
• Corrected iOS Button formatted text default color (it's different than the button with regular text)

BREAKING CHANGES:

• Line height will apply like a height as the name indicates while {N} has long been using it as line-spacing. This might cause strange visual changes in apps.