Initial skeletal implementation of the PrincipalService. Also, use the Login()/GetUserBlessing()

examples/bank_app/bank.mojom

Index: examples/bank_app/bank.mojom
diff --git a/examples/browser/browser_host.mojom b/examples/bank_app/bank.mojom
similarity index 64%
copy from examples/browser/browser_host.mojom
copy to examples/bank_app/bank.mojom
index 04d789b186681ba873bd874f59999a864f9022cc..e93481b1877522f2e51441fbdcb0736775744c61 100644
--- a/examples/browser/browser_host.mojom
+++ b/examples/bank_app/bank.mojom
@@ -4,7 +4,8 @@
 
 module mojo.examples;
 
-interface BrowserHost {
-  // Performs a navigation
-  NavigateTo(string url);
+interface Bank {
+  Deposit(int32 dollars);
+  Withdraw(int32 dollars);
+  GetBalance() => (int32 dollars);
 };