Add Wifi Settings Panel API

Wifi Panel is the fourth Settings Panel, which hosts Wifi related settings.
Currently the panel only holds the Wifi slice, but is open to future
additions.

Also add metrics constants for panel logging.

Test: atest SettingsPanelTest
Bug: 126894142
Change-Id: Ie72b3b52ea89047dfde77daa35ff2f95c655dbb7

• api/current.txt[diff]
• core/java/android/provider/Settings.java[diff]
• core/proto/android/app/settings_enums.proto[diff]
• proto/src/metrics_constants/metrics_constants.proto[diff]