Introduced short-term lock for UMS internal state

Added mUsersLock - short-term lock for internal state, when interaction and
synchronization with PM is not required. Modifications to mUsers and
mRemovingUserIds must be guarded by 3 locks: mInstallLock, mPackagesLock and
mUsersLock. While reads can use mUsersLock.

Testing revealed that the following methods in UMS often cause contention:
- exists
- getUserInfo
- getProfileParent

They all now use a short-term lock mUsersLock for reads.

Bug: 24979571
Change-Id: Ie3a22ea7cbb450c7969800fe2a4a2b2516165e5b