Specify keyboard metrics with physical unit "inch" (DO NOT MERGE)

This change also introduces the key background drawables which has no
fixed bottom padding.  Instead of relying on bottom padding in
drawable, this change also specifies Keyboard.verticalGap with
physical unit.  Other keyboard related length, distance and size are
also specified by physical unit.

Bug: 3066107

Change-Id: I44f3b2eef8086d6e0b0db53d38f08487549060c6