Allow setting values lower than MAX_ENTRIES_DEFAULT in max_cache_entries

Adjust the lower bound of max_cache_entries from
MAX_ENTRIES_DEFAULT(640) to 0. So we can do experiments with values
like 320.

Bug: 241953569
Test: atest resolv_unit_test
Change-Id: I4da710dccf4efe0edc625221b2519050069e0ba6