DeviceKeyHandle: The device should consume only known keys

When the device receive the key, only should consume it if the device
know about the key. Otherwise, the key must be handle by the active app.
Also make mDeviceKeyHandler private (is not useful outside this class)

Change-Id: I4b9ea57b802e8c8c88c8b93a63d510f5952b0700
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>