Fix support for USER_ALL in FakeSettings

If a content observer was registered to a SettingsProxy using
UserHandle.USER_ALL, testing that was expecting to pass would fail.

This CL fixes that by matching every user to USER_ALL observers that
match the Uri.

Test: atest SystemUITests
Fixes: 194934707
Change-Id: I41a0eb50a2cd1700b03b372bfcc199bebd9aea35