Fix getFontMetrics problems

Fixes a number of individual problems. In Minikin builds, gets base font
from TypefaceImpl rather than using whatever random font was in the
paint. Respects elegant metrics in ascent and descent methods as well as
getFontMetrics. Sets fixed ascent and descent values to 1900, -500,
which matches the Roboto font.

Part of the fix for bug 15467288 "Inconsistent line heights on
Minikin builds"

Change-Id: I12915f4a62de28932469c7279241eecf7320c47b