Optimized locking for get/setUserData

This change improves locking in the following methods:
 - setUserdata. cacheLock is only held when calling
   writeUserDataIntoCacheLocked
 - readUserData. dbLock is obtained only if not cached

Test: AccountManagerServiceTest
Bug: 36485175
Bug: 35262596
Change-Id: I65b939acedd69e3113c24b7e6788c7aefc6ba25a