Shell: Allow retrieval of wifi network credentials

This permission is needed for some wifi CTS tests.

Is the minimally scoped permission that needs to be added?
- Yes, this only allows the retireval of the saved wifi network
credentials on the device.

What options did you explore that did not need this permission?
- Possibly creating a UI to ask for manual entry of network credentials.
But, that would not let us do automated CTS tests (will need to become
CTS verifier tests then).

Bug: 150236894
Test: atest android.net.wifi.cts.WifiNetworkSpecifierTest
Change-Id: I6881003510191f667950be1ab27a5399fe3914b4