p970: make the G-Key actually useful and configurable

The G-Key will be remapped to App-Switch, and can be further configured in Settings->Keys like any other Softkey.

This commit includes:
 * Adding generic key layout with G-Key mapped to App-Switch
 * Including the new key layout
 * Tell the system there is an app switch key through overlay

Change-Id: I5c72ce32358f6423b4991d1247d6aa62516f7fd7