Add layout and keymap for Pixel C Bluetooth keyboard

This adds special layout and character map for Pixel C keyboard (Dragon
base) that lacks several keys, such as ESC, bracket keys, tilda and
backslash, which are mapped various Alt-R combinations.

Currently we map:

 [Shift]+AltR+2 -> ~ and `
 [Shift]+AltR+O -> { and [
 [Shift]+AltR+P -> } and ]
 [Shift]+AltR+= -> | and \

ESC mapping requires additional changes.

Bug: 24504154

Change-Id: I24fc68efe7e2ddebee26a273838443b441491857