Revert "Fallback to system's monospace font if theme excludes it"

This reverts commit 0f2cd788c837616b22a09de4c2620adad12b16cb.
Revert "Only force purge all when clearing singleton interface"

This reverts commit 4ce66cfe48d92241cbbdc5aaaa36b35baa647f2c.
Revert "New Theme Engine [3/6]"

This reverts commit c2807ac91187a30afe0e3e5755e2eabfdc9b9d1b.

Conflicts:
	include/ports/SkFontConfigInterface.h
	src/core/SkTypeface.cpp
	src/core/SkTypefaceCache.h