WifiManager: Add API to query STA + AP concurrency

Use the existing feature flag |WIFI_FEATURE_AP_STA| to expose a
@SystemApi.

PS: This flag is currently unused. Will need to find a mechanism by
which we can fill in this flag appropriately from HalDeviceManager.

Bug: 141452146
Test: Compiles
Change-Id: Idf3bab96686a8994cd3eea6bec8346db96161a03