Detect leaks on host by default.

Right now, leaks are disabled globally on host. This is problematic
because:
a). people write leaks in new code
b). if we fix leaks in old code, it doesn't get tested
c). fixing leaks in old code is harder because when we set
    ASAN_OPTIONS="" locally, other targets start hitting errors,
    and we have to set __asan_default_options there

Fixes: 141313466
Test: host ASAN build passes
Change-Id: I89298c7518c016e6a30884c75d7b791db16a5217