tracing: Add support for track event timestamps using custom clocks

Make it possible to write track events using custom timestamps together
with a custom reference clock. For example:

  TRACE_EVENT_BEGIN("cat", "Name",
      perfetto::TraceTimestamp{
          perfetto::protos::BuiltinClock::BUILTIN_CLOCK_MONOTONIC,
          timestamp_ns});

You can also use a completely custom clock id, but will also need
to manually emit a clock snapshot mapping that clock to the builtin
ones.

Change-Id: I2635eef84dc62368e6e529f43ac5ad9e751c601a