libcutils: Use proper private header to access C library's <time.h> extension

The strftime_tz() function is not a standard C library function and should not
appear in <time.h>. Instead, it is being moved to <bionic_time.h>, a private
header only available to platform libraries.

Change-Id: I4dc18939b9380725dfaa20c9e417296363bc52f4