Fix UserManagerTest#testSwitchUser.

ActivityManager.getCurrentUser() will return the targetUserId right
after ActivityManager.switchUser() is called. So, it is not a good
signal for the test to figure out if user switching is completed or
not. Instead just rely on the USER_SWITCHED broadcast.

Bug: 31045777
Change-Id: I58bff8a42ef4fa15c8667708f5956865b6417934