Use AccountCache#maybeGet instead of AccountCache#get

AccountCache#get returns an empty AccountState to represent a missing
account and the full name and the preferred email of the empty
AccountState are always null. Handle the missing account case explicitly
instead of using an empty AccountState.

Change-Id: I340d1b426304550ae9b368623e15291f8f1dbb92
Signed-off-by: Edwin Kempin <ekempin@google.com>