Use proptools.CloneEmptyProperties

CloneEmptyProperties is much faster than CloneProperties followed
by ZeroProperties.  Cuts >100ms off soong_build runtime, which will
become more important when we start calling soong_build a second
time to automatically generate documentation.

Change-Id: I45d948b36d9937a21b13dab068c3ce9a60b5a59b
1 file changed