Before this patch, the font fallback chain iterated all installed font
families if a variation selector was specified.

This CL narrows down the range of iteration.
To decide the font family for the variation sequence, we need to search
for both the variation sequence and its base code point.
The new range of the iteration is a union of them.

With this change, the running time of Paint.hasGlyph for the variation
sequence improves 50% and the running time of Paint.measureText for the
variation sequence improves 40% for the large text case on Nexus 6
userdebug.

Bug: 26784699
Bug: 11750374

Change-Id: Iced1349e3ca750821d8882c551551f65bb569794