Show RingtonePickerActivity in work user instead

Test: make RunSettingsRoboTests
Test: Try to import custom ringtone in work profile, no crash observed
Test: Set the custom ringtone as default. Tested both work gmail and
      calendar can play that ringtone.
Test: Try to import the peresonal ringtone, it is still working.

Bug: 37197930
Change-Id: I2b662247d7e70b59bae08e082e9682d74d797484