Add kill switch mechanism

This code implements the kill switch mechanism in the extension. This is
explained in //src/killSwitch/README.md and in the design doc:
https://docs.google.com/document/d/1O5YV6_WcxwrUyz-lwHOSTfZ3oyIFWj2EQee0VuKkhaA/edit.

Bug: twpowertools:64

Change-Id: Ia993c78035bba7038aafd53d156f20954217e86f
diff --git a/src/killSwitch/api_proto/README.md b/src/killSwitch/api_proto/README.md
new file mode 100644
index 0000000..ea2416e
--- /dev/null
+++ b/src/killSwitch/api_proto/README.md
@@ -0,0 +1,4 @@
+`*.proto` files have been retrieved from
+https://gerrit.avm99963.com/plugins/gitiles/gomodules/twpt-server/+/63f9486f7b00b35b05115a730648164864aa37cc/api\_proto/kill\_switch.proto.
+
+To generate the Javascript code, run `make grpc_proto_gen` in the project root.