Android: moving SecureRandom initialize to thread pool

Currently, the AsyncTask that creates the SecureRandom is on the
SERIAL_EXECUTOR - this is causing a lot of ANRs. Since it doesn't appear
to require any synchronus guarantees with anything else in the
SERIAL_EXECUTOR, we can move this to the THREAD_POOL_EXECUTOR.

This means when we block on SecureRandom creation using get(), we don't
have to wait on all tasks currently in the SERIAL_EXECUTOR finishing.

Bug: 866998
Change-Id: I771724a184bf501b2da55f7d42a93b3943e7a058
Reviewed-on: https://chromium-review.googlesource.com/1157211Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Sam Maier <smaier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579591}

chrome/android/java/src/org/chromium/chrome/browser/externalnav/IntentWithGesturesHandler.java

@@ -70,7 +70,7 @@ public class IntentWithGesturesHandler {
                 }
                 return secureRandom;
             }
-        }.execute();
+        }.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
     }
 
     /**