[WebApps] Hide Chrome icon in status bar with a persistent notification.

This is a revert of crrev.com/c/654118, as the decision has been made
to hide the status bar icon on pre-O Android. This is because on pre-O
users don't have the ability to selectivly silence notification
channels, so making the notification least intrusive is important.

Bug: 471623
Change-Id: If2efcdf680bba405fdf7c180119d6d6f331c38ab
Reviewed-on: https://chromium-review.googlesource.com/663105Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Piotr Swigon <piotrs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501489}

chrome/android/java/src/org/chromium/chrome/browser/webapps/WebappActionsNotificationManager.java

@@ -86,7 +86,7 @@ class WebappActionsNotificationManager {
                 .setShowWhen(false)
                 .setAutoCancel(false)
                 .setOngoing(true)
-                .setPriority(Notification.PRIORITY_LOW)
+                .setPriority(Notification.PRIORITY_MIN)
                 .setContentIntent(focusIntent)
                 .addAction(R.drawable.ic_share_white_24dp,
                         mWebappActivity.getResources().getString(R.string.share), shareIntent)