Use FLAG_IMMUTABLE on NetworkStackNotifier intents

FLAG_UPDATE_CURRENT was not helpful because:
 - For the venue info notification, the only extra is the Network, and
   the intent uses the network handle as identifier; meaning that two
   matching intents would already have the same extra.
 - The ACTION_WIFI_SETTINGS intent does not have any extra, so there is
   nothing to update.

Bug: 157475111
Test: atest NetworkStackTests; manual: connected to captive portals and
          used notifications
Change-Id: Ifd9e6ff04f2d574f3e28d612c8ea6215a209069e