[2/2] Settings: Hardware Key Rebinding

Possible mapping
 0 - Nothing
 1 - Menu
 2 - App-switch
 3 - Search
 4 - Voice search
 5 - In-app search
 6 - home
 7 - back
 8 - toggle last app
 9 - kill app
10 - Go to sleep

Device overlay required

overlay/packages/apps/Settings/res/values/config.xml
<!-- Hardware Keys -->
    <bool name="config_has_hardware_buttons">true</bool>

This is a port/merge of:
https://gerrit.omnirom.org/#/c/2474/
https://gerrit.omnirom.org/#/c/4659/

Also requires frameworks commit (https://gerrit.slimroms.net/#/c/2388/)
and overlay to function correctly

PS2/3: derp

Change-Id: Ie4794073236f0b6fe308b931d7d6e9d02a1ddf02
add in buttons icon for hardware keys