Unify multiuser switcher enabled setting

This does the following:
1. The multiuser switcher (in Settings and SysUI)
is now disabled by default. In order for it to be enabled
one of the following must be true:
  a. the user has explicitly toggled it on in Settings
  b. a new user gets created (via any means)
  c. config_showUserSwitcherByDefault overrides this default

2. Even if a new user is added, if the user had explicitly
*disabled* the switcher, the switcher still won't be enabled.

3. SystemUI and Settings (et al.) all use
UserManager.isUserSwitcherEnabled as the source of
truth in this regard.
No one else reads USER_SWITCHER_ENABLED directly.

4. If the switcher is enabled, then SystemUI will show the
switcher avatar, even if there are no other users on the device,
as long as new users can be created. This way, if the user
has enabled the switcher, the user can use the avatar to add
guest/secondary users (which would not be possible if enabled
status was tied solely to the existence of other users).

Bug: 137943217
Bug: 141372193
Bug: 149973281
Bug: 130270878

Test: manual: Settings > Multiuser doesn't turn on the systemui avatar
Test: manual: Settings > Multiuser is initially disabled
Test: manual: adb shell pm create-user A, does turn on sysui avatar
              even if the user didn't enable, but not if they disabled

Change-Id: Ia440b4db78792da76f94322a563d93db0c68e933