Fix new user setting in a call

In a call, the flag of UserManager is changed, then "Add user" and
"Guest" items in Settings are still enabled.
To fix this issue, these items will be disabled according to
"UserManager#canSwitchUsers".

Bug: 80447394
Test: make RunSettingsRoboTests ROBOTEST_FILTER=UserSettingsTest
Change-Id: Ib4c754603418528f5c6f1c78b8aabf33e863c8b9
2 files changed