Add setter for accounts in AccountsListAdapter

This allows the accounts to be set after the adapter has been created
which simplifies usage in cases where the accounts are loaded with a
loader.

Test: manually verify that account list is correct for default account
preference, import from .vcf and create label.

Bug 33627801

Change-Id: I5a33ad2746cff41d0251cead9a33dc53e5bf822d