Specify keyboard metrics with physical unit "inch"

Cherrypick I44f3b2eef8086d6e0b0db53d38f08487549060c6 from Gingerbread

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: Ib38355ebfc2f8d5b1d26c4e90eba87196fbeddfa