Trim spaces in the rule actions

When rule action contains something like
  cp $(FOO) ...
and FOO is set with
FOO := a \
 b \

the generated Ninja file constains extra spaces, making it
difficult to compare it to the same file generated by the
Starlark-based product configuration.

Bug: 181797530
Test: manual
Change-Id: I278bd8edf0f017a31c5b5115b2a38f4f663c55fc
diff --git a/core/main.mk b/core/main.mk
index 21f4387..cb97503 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -355,7 +355,7 @@
 ADDITIONAL_PRODUCT_PROPERTIES += ro.build.characteristics=$(TARGET_AAPT_CHARACTERISTICS)
 
 ifeq ($(AB_OTA_UPDATER),true)
-ADDITIONAL_PRODUCT_PROPERTIES += ro.product.ab_ota_partitions=$(subst $(space),$(comma),$(AB_OTA_PARTITIONS))
+ADDITIONAL_PRODUCT_PROPERTIES += ro.product.ab_ota_partitions=$(subst $(space),$(comma),$(strip $(AB_OTA_PARTITIONS)))
 endif
 
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