Enable prebuilt hiddenapi CSV files.

By enabling these hiddenapi CSV files to be prebuilt, it
becomes possible to create a split build that supports
the hiddenapi encode dex step, but doesn't contain all
of the java sources needed to generate the CSV files.

Bug: 175048716
Test: m nothing
Test: new TestHiddenAPISingletonWithPrebuiltCsvFile
Test: local build without prebuilt hiddenapi
Test: local build with prebuilt hiddenapi
Change-Id: Ia38c5016d2aeba54aa537a5ce601898d46330730
diff --git a/core/config.mk b/core/config.mk
index 9c04fe0..ed6429a 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -990,6 +990,13 @@
 
 endif # PRODUCT_USE_DYNAMIC_PARTITIONS
 
+# By default, we build the hidden API csv files from source. You can use
+# prebuilt hiddenapi files by setting BOARD_PREBUILT_HIDDENAPI_DIR to the name
+# of a directory containing both prebuilt hiddenapi-flags.csv and
+# hiddenapi-index.csv.
+BOARD_PREBUILT_HIDDENAPI_DIR ?=
+.KATI_READONLY := BOARD_PREBUILT_HIDDENAPI_DIR
+
 # ###############################################################
 # Set up final options.
 # ###############################################################