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

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.

Patch Set 13 - Some minor strings tweaks to fit with design guidelines

Change-Id: Iafeede4954f2632f5a97deee81d73924c8f95399