Synchronously decide if Android WebView should load a URL or not. (#1590)
This solves a class of issues when the WebView loses "context"
that a subsequent page load is the same as what was attempted
to be loaded previously. This solves a bug where a HTTP redirect
in combination with history manipulations causes a user to be
stuck and prevented from going back. Since WebView requests are
allowed to happen normally, debugging the WebView and tracking
redirects and page load initiators is more accurate and easier.
This will also bypass bridge latency and provide a faster navigation.
To do this, we must lock in the shouldOverrideUrlLoading callback
and send an event to JS. Currently, this callback is ran on
the main UI thread, of which we have no control over. This is
problematic as using the bridge in most ways seems to require
the main UI thread, which will cause a deadlock. However, using
BatchedBridge for Java->JS and a synchronous method for JS->Java
doesn't cause any problems. Additionally, it's been designed so
that if WebView suddenly runs the callback on a different thread
allowing for concurrency, it will continue to work.