Exposed some testAPIs in ActivityManager

- Added the following testAPIs to replace the usage
of adb shell commands in tests:
* stopUser and ACTION_USER_STOPPED
* waitForBroadcastIdle

- Changed the documentation for switchUser to correctly
reflect that it accepts CREATE_USERS

Test: verfied that tests can access the APIs
Bug: 181030374
Change-Id: I92b92471a18cf97a118d3f1946d6e06c277a76f1