Add CleanSpec for zygote changes.

default.prop is generated out of several makefile variables,
and it isn't possible to neatly express a dependency on those
variables (except perhaps by adding every possible makefile that
could define them to the deps list).

Change-Id: I93ea45c956aa22922f2389f735c058700621f674
Merge "Add CleanSpec for zygote changes."
auto-merger: merge in from aosp/master