Make UserTrackerImpl.userInfo thread-safe.

The userInfo property was not synchronized, which was causing crashes
in some cases. In the previous implementation userProfiles could change
and not contain the userId, throwing NoSuchElementException. This CL
makes the property thread-safe.

Bug: 363466988
Change-Id: Ibc5f9ea8d50f729e9d24b4647999350b4ea577a4
Test: None
Flag: EXEMPT bugfix