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
Merged-In: I612e32621b8045d085829507f22f6385b859f0c2
diff --git a/core/main.mk b/core/main.mk
index a061844..b412fe9 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -1247,6 +1247,7 @@
     $(CUSTOM_MODULES) \
   )
 
+ifdef FULL_BUILD
 #
 # Used by the cleanup logic in soong_ui to remove files that should no longer
 # be installed.
@@ -1267,6 +1268,7 @@
     $(test_files)))))
 
 test_files :=
+endif
 
 
 # Don't include any GNU General Public License shared objects or static