The U+20E3 COMBINING KEYCAP is used in our fonts to generate an emoji
rendering of ASCII numbers and letters through GSUB. For that to work
we need to choose the same (Emoji) font for the character coming
*before* the COMBINING KEYCAP character.
This is a special-case of a broader need to choose fonts per grapheme
cluster as opposed to per character, but for now, special-case U+20E3.
Bug: 7557244
Change-Id: I958e5a01068df8495bbb9bc3b9ed871cea1838b6
2f24599f4a