Introduce setFallbackTypeface

setFallbackTypeface is returned by Builder.build() method when the
provided font is not loaded due to some reasons.
The fallback family is resolved with width/italic passed to Builder.

Bug: 36794225
Test: android.graphics.cts.TypefaceTest passes
Change-Id: I65e220aca823fd815a52437b11c8e6dc952de8e2