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The UI thread is generally overbooked (esp at app startup time) so
avoiding it can greatly improve Cronet startup time.  The UI thread
was used for initialization previously because it simplified the
NetworkChangeNotifierAutoDetect and ProxyChangeListener logic because
BroadcastReceiver onReceived callbacks always happen on the UI thread
so those classes could be completely single-threaded.  This change
leaves those classes single-threaded except for their onRecieved()
methods which now immediately post to the new initialization thread
and check whether their BroadcastReceivers are currently registered.

BUG=709336
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_cronet_tester

Review-Url: https://codereview.chromium.org/2812963002
Cr-Commit-Position: refs/heads/master@{#469578}