Only use cacheLock when it's needed

When reading from cache, we can avoid synchronization on dbLock if we 
only read from cache (no db access).

When doing updates to db and cache, we should hold cacheLock only when 
updating the cache.

This change improves locking in the following methods:
 - getAccountVisibilityFromCache
 - saveAuthTokenToDatabase
 - getAccountsFromCacheLocked no longer allows outside locking. The 
   method was renamed to getAccountsFromCache and now self-manages locks
 - writeAuthTokenIntoCacheLocked
 - readAuthTokenInternal

Test: AccountManagerServiceTest
Bug: 36485175
Bug: 35262596
Change-Id: I9aca45c31716c4f0e0fd9f07859e88a7f5ba6922