Fallback to system's monospace font if theme excludes it

We should always have a monospace font available and if a theme
does not provide one the system's monospace font should be used.

Change-Id: Iaadf2f2c4d0dd7dd46d1eb58a687281440d70227
Merge commit '0f2cd788c837616b22a09de4c2620adad12b16cb' into cm-11.0