Insulate the account.db from updates.

I think we were getting accounts uninstalled on android devices when
SyncManagers called validateAccounts() while GmsCore was updating.

I try to prevent this by not removing accounts unless the data directory
has actually been removed (which shouldn't happen during updates).

Bug: 25611016
Change-Id: Ib526b3000ad741815d48fd020af2cc19d6205166