Changes on am / cmd user for ITestDevice integration

- Added (optional) listener to startUserInBackgroundVisibleOnDisplay
- Added cmd user is-visible-background-users-supported

It will used by shell command and ITestDevice (which uses the cmd).

Test: adb shell cmd user is-visible-background-users-supported
Test: adb shell am start-user --display 2 -w 10
Test: atest -it com.android.server.am.ActivityManagerServiceTest UserControllerTest

Bug: 266851112

Change-Id: I04648a0ba91ec883273cf519c77b081eabfd5187