Update UserManager#removeUser to follow API guidelines.

Updating the javadoc to explain the return value and adding in a null
check for the UserHandle passed in.

Bug: 124883455
Test: atest com.android.server.pm.UserManagerTest#testRemoveUserByHandle
Test: atest com.android.server.pm.UserManagerTest#testRemoveUserByHandle_ThrowsException
Change-Id: I1c2a7e140544001d6341b423890281ce53d7b38c
3 files changed