New TestAPIs on ActivityManager to not stop bg users on switch.

Test: atest NeneTest:UsersTest
Test: m update-api
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

Change-Id: Ib57989aff323dc1f7d98720d01215e4f7c79ba3a