Allow disabling of CleanSpec functionality

This is only intended for testing purposes -- so that we can run build
system tests and verify that kati only gets run once for each
configuration. CleanSpecs necessarily write state that require us to
re-run kati twice.

Test: export NO_ANDROID_CLEANSPEC=true; rm -rf out; m -j; m -j
Change-Id: I635a8d8b6754dff82fbc7f20c500d06d7cf6c4f2