atrace: avoid unnecessary writes to trace_clock

Writing to trace_clock erases the trace buffer, even if the value
hasn't changed.  This prevents use of --async_start and --async_dump
to leave background tracing running and dump after an even that
needs debugging, because --async_dump writes to trace_clock and
resets the buffer before it can read it.

Read and parse the current value from trace_clock before writing,
and skip the write if the value isn't changing.

Change-Id: Ia2ec5bb654fb0bd179771b511ff261731ba47dca