Added options for custom search key and long-press search key app.

- added an option to choose a custom app for search key in
  input settings
- added an option to choose a custom app for long-press
  search key in input settings
- fixed a bug where volume control option was disabled on
  devices without camera button and trackpad (stumbled over it)
- cleaned up stings.xml a bit

Change-Id: Ic7be975a5bced410c56527d88af95d33ac9a1ce4