Introduced additional lock - dbLock

Right now it is always used with cacheLock. In the future, we will be
adding optimizations to reduce cacheLock contention by only holding it
when updating the cache.

This change is non-functional and doesn't change the current locking
contract

Test: Manual + AccountManagerServiceTest
Bug: 36485175
Change-Id: Iebc437463958d33b32fc1273a84680c22ac60825