Build build_flags.json in soong

Define release_flags_json module type to install build_flags.json in
'etc'. release_flags_json reads the json files generated from the
soong release-config command.

Bug: 324996303
Test: build and see if the files are installed
Change-Id: I8cdcb7c61dd75cc54e4912d2ed7d1687f424151c
diff --git a/aconfig/build_flags/Android.bp b/aconfig/build_flags/Android.bp
index 69e4316..b3c7339 100644
--- a/aconfig/build_flags/Android.bp
+++ b/aconfig/build_flags/Android.bp
@@ -14,6 +14,7 @@
     ],
     srcs: [
         "all_build_flag_declarations.go",
+        "build_flags.go",
         "declarations.go",
         "init.go",
     ],