Fix Backslash not showing with paired keyboard

Our current label retrieving flow is

1. Special cased drawables (e.g. backspace)
2. Special cased names (e.g. F2)
3. Device KeyCharacterMap getLabel

For single character keys such as '\', we reach the end of 3. with
no result if the main device doesn't have the key. This change uses
the virtual keyboard map as a backup to retrieve labels for these
last resort cases.

Bug: 28830189
Change-Id: I69cc82a11e7b803db7f60ea07d34c59f6000149e