Add "Work profile" section to Languages and input settings

This CL adds "Work profile" section to Languages and input settings.

The new section is visible when and only when work profile is enabled.
Hence there should be no user-visible behavior difference when work
profile is not enabled.

If per-profile IME [1] is also enabled, the following 3 preferences
will be shown in the new section.

 * Virtual keyboard for work
 * Spell checker for work
 * Personal dictionary for work

 [1]: Ied99664d3dc61b97c919b220c601f90b29761b96
      a878b9500e6b89dce9738179edc27bcd0d736b7e

Fix: 120748696
Test: Manually verified as follows.
  1. Build and flash an AOSP build
  2. adb root
  3. adb shell setprop persist.debug.per_profile_ime 1
  4. adb reboot
  5. Install Test DPC
  6. Enable managed profile with Test DPC
  7. tapas SoftKeyboard && make -j
  8. adb install -r --user 0 $OUT/system/app/SoftKeyboard/SoftKeyboard.apk
  9. tapas SampleSpellCheckerService && make -j
 10. adb install -r --user 0 \
     $OUT/system/app/SampleSpellCheckerService/SampleSpellCheckerService.apk
 11. Open Settings -> System -> Languages & input
 12. Virtual Keyboard -> Manage keyboards:
     Make sure both "Android Keyboard" and "Sample Soft Keyboard" are shown.
 13. Spell checker -> Default spell checker
     Make sure "Android Spell Checker" and "Sample correction" are shown.
 14. Advanced -> Virtual Keyboard for Work -> Manage keyboards
     Make sure that only "Android Keyboard" is shown.
 15. Advanced -> Spell checker -> Default spell checker
     Make sure that only "Android Spell Checker" is shown.
Change-Id: Id88928318ab1cd92963d652ae39ca772222ea4ac