liblog: logprint supports number of seconds time event field

Add s to report time in seconds.  The time could be a period, duration
or monotonic, expanded to seconds, minutes, hours and days.  gTest has
to acquire a dynamic tag allocation as there are no users of this
feature yet.

Looking to the future, audio media logging has binary content similar
to the binary events structures Android logging uses and they have
a definition of a duration field in their internal binary logging, so
may be of use when we unify the logs.

Test: gTest logcat-unit-tests --gtest_filter=*.descriptive
Bug: 31456426
Change-Id: I262c03775983b3bc7b1b00227ce2bb2b0f357bec