[ermine] Set flutter_driver_extendable using build arg.

Set the flutter driver enabling flag from a build argument instead
of hardcoding it. In debug build, flutter driver is enabled, but it
does not allow the app using it to consume keyboard input unless it
is coming from test scripts. This change allows using the app using
real keyboard input during development. To test using flutter driver
please set:
  fx set workstation.x64 --args=flutter_driver_enabled=true

Change-Id: Ic12515b00cf1127b8987c933ad11065a68cd98cc
diff --git a/session_shells/ermine/shell/BUILD.gn b/session_shells/ermine/shell/BUILD.gn
index e9cce34..e1c57a3 100644
--- a/session_shells/ermine/shell/BUILD.gn
+++ b/session_shells/ermine/shell/BUILD.gn
@@ -15,13 +15,18 @@
   ]
 }
 
+declare_args() {
+  # Enable flutter_driver builds. This is effective only on debug builds.
+  flutter_driver_enabled = false
+}
+
 flutter_app("ermine_app") {
   main_dart = "lib/main.dart"
 
   fuchsia_package_name = "ermine"
   package_name = "ermine"
 
-  flutter_driver_extendable = true
+  flutter_driver_extendable = flutter_driver_enabled
 
   meta = [
     {