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