Support dead characters when key presses are shown

Got a crash while testing the German layout, and it seems like the
current "show key presses" functionality will crash when using
an unsupported Unicode character. This CL addresses this issue by
adding support for dead characters, pulling the actual combining
character from the KCM if needed.

Bug: 325420862
Test: Flashed on device
Flag: NONE
Change-Id: Ia70685fbfae5bf81a71b346a24ff0e73c8d8d932