Create API for CarProgressBar stroke cap

Created new CarProgressBarStyle class to encapsulate all style-related properties of CarProgressBar.
Moved the color property to CarProgressBarStyle and added new strokeCap property there.

Test: Added unit tests for CarProgressBarStyle

Relnote: Added support for custom stroke cap to `CarProgressBar`

Bug: 543336954

Change-Id: I17141d3ad7bfa9440b058ad7f2e7ea1fc0ff4500