Don't touch .installable_files for non-FULL_BUILD

If we're not doing a full build, leave
$(PRODUCT_OUT)/.installable_files* alone rather
than truncating it. This prevents the clean logic
in Soong from removing everthing that we're trying
to use to build an image using the snod, vnod, etc.
goals.

Bug: 149460609
Test: make droid, make vnod, verify vendor.img
Change-Id: I612e32621b8045d085829507f22f6385b859f0c2