Rewrite HTTP auth dialog code

The old code reached into the AlertDialog view using hidden
view ID constants to pull out the title. I've moved the dialog
to its own class and made it only use public APIs.

Change-Id: I84f576ece09ca802091175e1f928ae0be4a35d07
diff --git a/src/com/android/browser/Tab.java b/src/com/android/browser/Tab.java
index cb70e01..36566c1 100644
--- a/src/com/android/browser/Tab.java
+++ b/src/com/android/browser/Tab.java
@@ -818,8 +818,7 @@
                 handler.proceed(username, password);
             } else {
                 if (mInForeground) {
-                    mActivity.showHttpAuthentication(handler, host, realm,
-                            null, null, null, 0);
+                    mActivity.showHttpAuthentication(handler, host, realm);
                 } else {
                     handler.cancel();
                 }