settings: Various cleanups

 * Replace the single-option menu with a switch in cast screen
 * Use a seekbar preference to allow setting arbitrary animation scale
values

Todo (depend on outstanding draft patches) :

   - Remove Lockscreen wallpaper option from Lockscreen settings
       - Use new ThemeChooser instead
   - Kill Trebuchet sub-settings from Home settings

PS1: keep default on .75 & remove unneeded arrays

Change-Id: I4ceaf0dc74e6e044e36cf3652bfc1efb33890380