DO NOT MERGE Fix account list for group editing

- Don't use contact writable account list to populate the
account picker when creating new groups, instead check for
isGroupMembershipEditable() on the account type

- Repeat the same for determining whether the "new group"
button should appear in the PeopleActivity or not

Bug: 5360120
Change-Id: Ia1ec62eff3fe4fd5b495548218c58f238ff49788