CB-5988 Allow exec() only from file: or start-up URL's domain

Uses prompt() to validate the origin of the calling JS.
This change also simplifies the start-up logic by explicitly disabling
the bridge during page transitions and explictly enabling it when the
JS asks for the bridgeSecret.

We now wait to fire onNativeReady in JS until the bridge is initialized.
It is therefore safe to delete the queue-clear/new exec race condition
code that was in PluginManager.