Fix clang-tidy warnings in log_time.h

 - Zero initialize log_time instances by default
 - Disable implicit conversions by making constructors explicit
 - Explicitly initialize to EPOCH in most places
 - Change sniffTime() to avoid in-place modification of a log_time

I stopped here, but we could consider following up with a more invasive
change to make log_time instances immutable and perhaps also remove the
default constructor to force explicit initialization to EPOCH.

Test: atest libbase_test netd_unit_test
Change-Id: I67e716ef74adaaf40ab2c6e2e0dddb8d309bc7ca