Hide implementation details of log_time struct

In the future, the sizes of tv_sec and tv_nsec (or even the size of
log_time struct itself) can change due to the 32-bit overflow expected
to happen in the year 2138. In order to hide such implementation details
to the clients of liblog, the two macros LOG_TIME_SEC and LOG_TIME_NSEC
are introduced.

Furthermore, vendors are provided with a simplified version of log_time.h
without C++ APIs. In doing so, log_time.h no longer includes time.h.
This breaks several modules that implicitly relied on the hidden
dependency, which should be fixed.

Bug: 37629934
Test: build with BOARD_VNDK_VERSION=current
Change-Id: I01b36078c1d8f3f44824be20ae769ba1465b6feb