[Sb refactor] Default SSID for DemoWifiRepository

Increase the usability of the wifi demo mode repository by defaulting
the `ssid` to "Demo SSID" when none is provided.

Test: adb shell am broadcast -a com.android.systemui.demo -e command network -e wifi show -e activity inout
Test: verify that the inout indicators are showing in demo mode
Bug: 269124538
Change-Id: Iea93f0c868faf27a004d909a37b6705caf0f4ebf