Modify content descriptions of star and search icon

Using existing strings so we don't need to bother i18n team.

Bug: 32972143
Bug: 32973650

Test: manually verified star icon in all contacts list is announced
      as "Favorites" and search icon is announced as "Search contacts"
      in contacts list and in contacts selection (to add contacts to
      group).

Change-Id: I3df125435f864a3a769e85c4107ff594a7834bc1