Restrict visibility of EXTRA_INSTALL_ZIPS

After aosp/3089654, Android.mk files will be able to read
EXTRA_INSTALL_ZIPS, which we don't want. Use the new
$(KATI_visibility_prefix) to restrict its usage.

Test: m nothing passes, adding $(warning $(EXTRA_INSTALL_ZIPS)) in build/make/core/main.mk produces an error
Change-Id: Ib3c0079efd534e580a176fc69f746852b66297af