Fix test failure due to missing Roboto VF font

Due to b/189235998, the weight 400 of default font is no longer a
VF font. To be able to test the interpolation of variable font,
create Typeface manually for testing purposes.

Bug: 190623361
Test: atest TextInterpolatorTest
Change-Id: I4ef5f5b98f8f5af75e5c30121ace0e2d9b30a649