Dynamically return the uri of WifiSlice.

WifiSlice is used by both ContextualWifiSlice and WifiSlice itself.
In contextual homepage, we provide contextual_wifi slice for the
homepage. So in our slice pre-check, contextual_wifi slice will gets
pinned and bound. But when it comes to the state where wifi is on but
not connected, it actually returns wifi slice instead of contextual_wifi
slice, which will hit slice not pinned exception as wifi slice never
gets pinned.

Thus, we have to dynamically return the corresponding uri to  avoid this.

Fixes: 124627340
Test: robotests
Change-Id: I54ba255871628bb8eb814442f990a5d1149461e0