Improve FloatingActionButton border implementation

The previous impl had overdraw issues as it draw a whole
oval behind another large opaque oval. This implementation
moves us to a custom Drawable which draw a stroke oval.

It also allows us to move to a custom LinearGradient shader
to more closely match UX's spec.

Change-Id: I93f2c3c444d0a23fd542f2a35ac1d43d4f699bd6
8 files changed