UserManager: move preCreate code to new method

This cl just moves the block of code that is dedicated to converting
preCreated users to its own method to make the code look less messy.

Also adds synchronization to the writeUserLP calls within that block.

Bug: 142482943
Test: Treehugger (this cl just moves some code into a new method)
Change-Id: I7326a6559540b7166f737a9ecc7a5745ffc67554
1 file changed