[Android] Use per-renderer message filter instead of per-WebView
Address a long-standing TODO for using a single browser message filter
per renderer process (RPH), instead of installing each
GinJavaBridgeDispatcherHost as a filter (that is, for each WebView).
This also required factoring out JavaBridge thread into a separate file,
which has trimmed down the contents of gin_java_bridge_dispatcher_host.cc.
Review URL: https://codereview.chromium.org/
1262203002
Cr-Commit-Position: refs/heads/master@{#341360}