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

This reverts commit 7a0a1d3d77aa810bfda218bb3e2961f43596ae03.
Revert "Only force purge all when clearing singleton interface"

This reverts commit 6af0ebc23735dc9e7d61aa660928f19419b0a916.

Conflicts:
	src/core/SkTypefaceCache.h