Add Wifi Slice

Add a custom Wifi Slice to the Settings Slice Provider.
It needs a custom Slice because of the complicated listener logic
in the MasterSwitchPreferenceController, which makes it hard to
work-in synchronous set/get logic.

The one-off Slice requires extra changes, including:
- Including it in getDescendants
- Handling changes to wifi by the framework

This is the first change that uses SettingsLib's broadcast relay,
which allows settings to (un)register IntentFilters to a Uri,
allowing Settings Slices affected by the framework (quicksettings,
connectivity related, volume, etc) to be updated without action
on the Slice.

Fixes: 70622039
Fixes: 67997332
Test: robotests
Change-Id: Ia76738dd6baacd5522d52df2c61ebad86a600282
Merged-In: Ibfe4736beecb833e3f6bb871b2eb5228a5fd3a34