Expose stopAppForUser in ActivityManger

Adding wrapper function to call stopAppForUser from
ActivityManagerService. It can be used stop app when
full scale 'force stop' is not required.

Flag: EXEMPT bug_fix
Test: m && boot device
Test: m Settings && adb install -r $ANDROID_PRODUCT_OUT/system_ext/priv-app/Settings/Settings.apk
Bug: 389696304
Change-Id: I34e8720e2f227c7f93e2e9e1df7db993aa81c607