LockScreen: allow long-press actions from hardware keys (2/2) - Slim

        Allow following actions with button long-presses on lockscreen:
        - toggle flashlight (if available)
        - play/pause/next/previous track selection
        - toggle silent mode

        Allow customizing of back, menu, and home buttons. Search
        button events are completely ignored on the lockscreen.

        Auto-detect hardware keys, only show appropriate menu
        options based on the actual hardware keys available on the
        device (or completely hide if there are no hardware keys).

        Based largely on Danny Baumann's work from gingerbread.

        Ported from CM10

        Change-Id: I36ea5eb7e93d68de872536dc6d7e93f99ba1621f