Refactor keyboard layout unit tests

- Move predfined keys definitions to AbstractLayerBase and make it a
  top-level class.
- LayoutCustomizer extends AbstractLayoutBase that provides unit tests
  to access helper method and predefined key easily.
- Move shared layout custmizers to new layout.customizer package.
- Move private layout customizers to layout classes.

Change-Id: I9421cef1bedf5759fb0434842ac13a3fe3320b69