Renamed setStopBackgroundUsersOnSwitch to setStopUserOnSwitch()

Test: atest NeneTest:UsersTest
Test: m update-api CtsVerifier
Test: adb shell cmd activity set-stop-user-on-switch false
Test: adb shell cmd activity set-stop-user-on-switch
Test: adb shell dumpsys activity users|grep OnSwitch

Bug: 203752848

Merged-In: Iea03164319768abf159bf8ae2ccc539620cb30fe
Change-Id: Iea03164319768abf159bf8ae2ccc539620cb30fe