Note when we don't know a default phoneId

Otherwise we don't notice the new default on sim swap.
On a single-sim device the default phoneId will be
the same again but if we don't notice it's loss
we won't notice its gain.  The pattern:
initial condition
phone[0] subId 1
default subId 1

sim removal
subId changes 1 -> -2 (invalid)
    this causes both subId notification and default notification
    prior to this change this did both, but left the SubMonitor
    thinking the default phoneId was still 0.  After this CL
    it changes that to INVALID

sim insert
subId changes -2 -> 2
    this causes a subId change notification
default changes 1 -> 2
    this should cause a default notification, but before this CL
    we though the default phoneId was 0 and now it's again 0, so
    no notification.  With this CL it'll be a INVALID -> 0 change
    and the notification will go out

bug:27425942
Change-Id: I2875a223fbf298d23037875302e25816fe19a52e