Adaptive keyboard width/position parser

This change introduces the following features to Keyboard XML format.

 * "keyXPos" can specify the key X coordinate directly.
 * "keyXPos" can be negative. The X coordinate will be calcluated from
    the right edge of the keyboard toward left.
 * "keyWidth" can be zero to be filled up to the right side.
 * "keyWidth can be negative. The key will be filled up to both sides.
 * Spacer's horizontalGap is renamed as keyWidth, and can be inherited
   from key-style.
 * Spacer can have keyXPos attribute.

Using these syntax, all keyboard layouts have been re-written.

Cherry-Pick: I314b2e8ca2aa145ff9506cbf927140a15685af42
Bug: 4442045
Change-Id: I048fe5eaef020d8472ab577e9d326042bae2f3fa