Drop frames based on presentation timestamp

If there are two or more buffers pending that are ready for
immediate presentation, drop all but the last one.

Any code that didn't explicitly specify timestamps for buffers
was using the default value (auto-generated "now").  As a result,
surfaceflinger would drop frames whenever more than one buffer
was queued.  We now use zero as the auto-generated timestamp,
and we don't set the timestamp in eglBeginFrame().

Change-Id: I187f42d33de227cd3411ff0dcd3b9ce1961457eb