Make Paint a required field of StaticLayout Builder

This patch moves the Paint parameter from being an option set from
setPaint to being a required field passed into the obtain() method
of StaticLayout.Builder. Thus, it is now possible to get a valid
StaticLayout object just doing .build() on the Builder; all other
fields are optional.

This is in preparation for exposing the builder publicly, but that
is to be a separate CL.

Bug: 20190561

Change-Id: Iefd7800203004e565d068b0c76502cf7cf52d91f