Fix Dialer OOM when building smart dialling index

If a contacts database had contacts comprising an unreasonably large
(> more than 50) raw contacts, querying for these lookup keys was likely
to cause problems.

Bug: 13133579
Change-Id: I26334dfce583eeaa1c7b8db7c03b7f847534de1b
(cherry picked from commit e3c7e425)