Make "up" button different from "back" button

- Explicitly go to PeopleActivity when "up" is pressed
- Use flag FLAG_ACTIVITY_CLEAR_TOP to prevent having
PeopleActivity multiple times in the back stack
- Use PeopleActivity.class because we don't want to use
a URI like LIST_ALL_CONTACTS otherwise it will switch
the tab or the account filter that has been last set
by the user

Bug: 5071475
Change-Id: Id6f768db3838fa0bc007feda89a92e3d36c8a694