Add custom hardware button mapping [1/2]

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

Overlay required

depending on the number of buttons on your device

<!-- Hardware 'face' keys present on the device, stored as a bit field.
     This integer should equal the sum of the corresponding value for each
     of the following keys present:
         1 - Home
         2 - Back
         4 - Menu
         8 - Assistant (search)
        16 - App switch
     For example, a device with Home, Back and Menu keys would set this
     config to 7. -->
     <integer name="config_deviceHardwareKeys">7</integer>

Needed by: https://gerrit.omnirom.org/11117

Change-Id: Ie28bda05e80f5c3bfc63c35c0ade8b0875e85e97

Conflicts:
	core/res/res/values/config.xml
	core/res/res/values/du_symbols.xml
	core/res/res/values/symbols.xml
	policy/src/com/android/internal/policy/impl/PhoneWindowManager.java