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
diff --git a/liblog/log_time.cpp b/liblog/log_time.cpp
index ae376be..77bb94f 100644
--- a/liblog/log_time.cpp
+++ b/liblog/log_time.cpp
@@ -137,7 +137,7 @@
 LIBLOG_ABI_PRIVATE log_time log_time::operator-=(const timespec& T) {
   // No concept of negative time, clamp to EPOCH
   if (*this <= T) {
-    return *this = EPOCH;
+    return *this = log_time(EPOCH);
   }
 
   if (this->tv_nsec < (unsigned long int)T.tv_nsec) {
@@ -165,7 +165,7 @@
 LIBLOG_ABI_PRIVATE log_time log_time::operator-=(const log_time& T) {
   // No concept of negative time, clamp to EPOCH
   if (*this <= T) {
-    return *this = EPOCH;
+    return *this = log_time(EPOCH);
   }
 
   if (this->tv_nsec < T.tv_nsec) {