Allow SF traces to omit default values on the property list

The C++ proto2 implementation write the default values ot the file.
Until the surface flinger dump is updated to proto3, this implementation
will not have any effect.

Test: (Un)Check the `display defaults` checkbox. Load a file. If the
checkbox is unchecked, default values should not appear on winscope,
otherwise they should appear.

Change-Id: I2432966f3e14aba5fc1dee26d5a09c743406b417