Add support for fallback keycodes.

This change enables the framework to synthesize key events to implement
default behavior when an application does not handle a key.
For example, this change enables numeric keypad keys to perform
their associated special function when numlock is off.

The application is informed that it is processing a fallback keypress
so it can choose to ignore it.

Added a new keycode for switching applications.

Added ALT key deadkeys.

New default key mappings:
- ESC -> BACK
- Meta+ESC -> HOME
- Alt+ESC -> MENU
- Meta+Space -> SEARCH
- Meta+Tab -> APP_SWITCH

Fixed some comments.
Fixed some tests.

Change-Id: Id7f3b6645f3a350275e624547822f72652f3defe