fixes mStartedUsers shoule be locked by mLock, not mVolumes

when create/delete multiple user, plug/unplug externel storage(such as sdcard) at the same time.
multiple threads run concurrency , the newly created user can not get state of externel storage.

https://code.google.com/p/android/issues/detail?id=215901



Change-Id: I097f5291108af89a430e12045376065d71d48175
Signed-off-by: yuanhuihui <yuanhuihui@xiaomi.com>