p2p: Close the channel when onStop is called.

P2P dialog is moved to an indenpendent activity, it will push Settings
to background and trigger onPause(). As P2P dialog is a Dialog Activity,
it won't trigger onStop, but SoftAp and NAN activity will. Moving
channel closing to onStop to adapt to new P2P dialog design.

Bug: 219406778
Test: make RunSettingsRoboTests ROBOTEST_FILTER=WifiP2pSettingsTest
      create a p2p connection between 2 devices with this build.
Change-Id: I2fab5aa1021ec1a993f811c7310079db9d7f03c0