Add Settings.Global.Wearable to pre-defined keys list.

This allows wear hidden settings to follow the @Readable annotation
restrictions as described in
https://g3doc.corp.google.com/company/teams/android-framework/android-packages/settings-provider/readable/index.md?cl=head

Bug: 283855901
Test: manually tested that 3p apps lost access to @hide wearable
settings.
Test: atest SettingsProviderTest

Change-Id: I3f7c1f51e2aa7ab93ce86ec929b62e587c150873