Handle placeholder characters during translation

Bug: 21131556

If we fall behind, initially use blanks as placeholder characters.
Convert these to the right character (currently a correctly sized
space) during translation.

This avoids log spam by the translation code with minimal complication.
It also seems a bit cleaner.

Add some missing private qualifiers in the same vicinity.

Change-Id: I9c53d74ce2a234c888cee55ba72575c884f18cae