Add lightweight visual indication for numbers.

This change adds lightweight visual hints of alternate numeric characters on the top row - e.g. having a light gray '1' on the upper right corner of letter 'q'

Note that MDPI resources are tentative (the same as HDPI for now, until we get fixed MDPI visual assets).

bug: 3004632
Change-Id: I7a25cf90b702433a844c88f5c47bf914706af9bc